She was about to pull the kind of move that prosecutors dream about, but no smile gave her away. No strut marked her walk to the well of the court, no tiny concession to triumph breached her reserve. At one of the biggest moments in the biggest case in town, Judith E. Retchin examined her witness as though she were asking directions to the bus.

Darcelle Walker, the witness, could not have been more skittish if she were standing trial herself. The better to spring her unawares on the defense, prosecutors had served her with a subpoena that very morning. Come to U.S. District Court, they commanded her, and testify against Mayor Marion Barry.

As it dawned on some of the spectators what the woman intended to say, the galleries began murmuring in amazement: Walker was here to settle a disagreement. Prosecution witness Lydia Reid Pearson had testified earlier that she gave crack cocaine to the mayor along with a job application on Sept. 7, 1988. But the defense then called a witness to undermine Pearson's claim. Clifton A. Roberson, a city official, testified that it was he who accepted Pearson's application, and no drugs accompanied the form.

When she calmed down enough to testify, Walker said only this. She had worked in Roberson's office that day. She was responsible for time and attendance sheets. And she typed a D.C. government AWOL form -- now prosecution exhibit 104 -- marking Roberson's unexcused absence from the job. The hours when Roberson apparently was absent, from 8:30 to 10:30 in the morning, were the very ones when Pearson had been there with her application.

Think of the odds! That the mayor's best alibi witness would turn up AWOL. That his absence would leave a paper trail. That prosecutors would find the thing, and bring a witness into court to tell about it. Half tactical wizardry and half blind luck, it was one of those moments that can make or break a trial. Reporters scribbled. Spectators marveled. The mayor turned to Anita Bonds -- Roberson's immediate boss on the day in question -- and fixed her in a gaze that was nothing like pleasure.

Only Retchin, the assistant U.S. attorney, betrayed no reaction. She checked off a line on a long yellow pad and posed her next question to the witness. Another rabbit, another hat, another day in court for the prosecution.

For six relentless weeks it has been like this. No one should have been surprised that Retchin and her partner, Assistant U.S. Attorney Richard W. Roberts, would lay out a damning narrative against the mayor. Assembling evidence and deploying it are what prosecutors do for a living.

What surprises, and impresses, is the talent they have revealed as counterpunchers -- an art more often practiced by the defense. Not once in 28 days of trial did R. Kenneth Mundy, one of the city's wiliest courtroom tacticians, catch Retchin or Roberts seriously off guard. Anticipating every thrust, they have been ready at critical moments with startling parries. In the process the two young prosecutors have turned some of the old veteran's best moves against him.

What is more, they have behaved with a self-restraint that borders on self-denial. Though neither learned the trade under Jay B. Stephens, Retchin and Roberts are models of the style he promotes as U.S. attorney: emotionless and methodically aggressive, submerging all signs of personality in the role. Accordingly, they do not cooperate in the preparation of newspaper profiles.

For Retchin that detached approach comes naturally. For Roberts, according to some friends, it has taken some adjustment.

But for a low hum from the air vents, the courtroom was entirely without sound -- no shuffling, no breathing. On opening day of the Barry trial, Roberts had transfixed Courtroom 2.

Whose idea was it for him to pause in his opening statement for a silent demonstration of Marion Barry's crack-smoking technique? Did he practice in front of a mirror, using the Vista Hotel videotape like a home instructional aid? Could Stephens, watching over his most important trial as U.S. attorney, possibly have endorsed the risks involved in such a stunt?

Just a few minutes earlier, Roberts had lounged pensively at the prosecution table, leaning back in his big leather chair and sipping from a paper cup of water. His wife, Vonya B. McCann, a partner at the law firm of Arent, Fox, Kintner, Plotkin, & Kahn, sat in the first row of spectators. Stephens, in the VIP row, wore the red, white and navy blue of official Washington.

Roberts consorted with neither. After 18 months of prelude, the curtain was going up.

"You may proceed," said Judge Thomas Penfield Jackson, and Roberts stood.

His language was precise, vivid, almost novelistic. There were scarcely any adjectives in his opening, and no abstractions: only sharp, detailed images of the mayor's alleged criminal behavior. The rocks of crack inside a matchbox. The corner of a business card used to raise some powder cocaine to the mayoral nostril.

Roberts spoke evenly, without theatrics. When he wanted to emphasize a point, he slowed his delivery a beat and enunciated like an elocution coach. "You will hear him lie," he declared, speaking of the mayor's tape-recorded testimony before a grand jury.

Then came the memorable pantomime.

"You will see the defendant with your own eyes on black-and-white videotape," Roberts said, "raising to his mouth a small crack pipe stem that he had loaded with little rocks of crack cocaine, lighting it with a cigarette lighter and smoking crack, just like this."

Roberts cupped his hands around an imaginary pipe, tilting his head forward and then leaning back into the draw. He could have been playing a saxophone, but the only sound was a whistle of air between his teeth. He held the imaginary hit eight seconds, nine seconds, 10 -- an eternity -- then blew it out.

"Then raising it a second time to finish off what he had, just like this."

And Roberts did it again! He seemed entirely un-self-conscious. In a dozen ways the demonstration could have gone awry, but Roberts pulled it off without a false note. It is hard to conceive that Retchin would have tried.

The telephone rings at Roberts's house. It rings again, and is answered by a machine.

A thumping rap beat announces the recording. A man's deep voice drives the cadence -- BOOM-shhhhk BOOM, ba-BOOM-shhhhk BOOM -- as a woman, stifling laughter, raps the words:

"Hi, this is Ricky, and Vonya too/ We can't come to the phone to greet you/ We wish you would leave a message at the tone/ And we'll give you a call when we get back home."

BOOM-shhhhk BOOM, ba-BOOM-shhhhk BOOM.

Can that really be Roberts on rhythm? Can that be his wife, Vonya McCann? Is that any kind of greeting for a federal prosecutor?

Roberts, who turned 37 during the Barry trial, is full of surprises. If "every person has two sides," as Roberts said of Barry in his opening statement, the private Ricky Roberts shows signs of a rich existence beyond his job.

He's a hot jazz pianist in an office that can barely hum a tune. His politics would seem more in character in a public defender. And in a case in which the government is charged with pursuing a racist vendetta, he's an African American who made his career putting racists behind bars.

"He's a real kind of a cause guy, an artsy lefty," said one good friend, whose job does not allow him to be identified. "It's a surprise to see Ricky up here as a prosecutor."

At New York's High School of Music and Art, Roberts was a serious student of the piano. At Vassar, in the path-breaking class of '74 -- the first to admit men -- he spoke out for a less Eurocentric curriculum. In law school at Columbia, he marched for divestiture from South Africa, displayed an interest in African socialism and expressed dissent from U.S. policy toward Cuba by traveling there.

"Did Ricky wear dashikis? Of course, but does that mean people who wear dashikis don't make good prosecutors?" said Eric Washington, a law school friend and now a partner in private practice here. "I wore dashikis. A lot of people wore dashikis."

If Roberts meant to go into the business of prosecuting, it seemed natural that he would turn to civil rights. But even in the Carter Justice Department, where he had his first job out of law school in 1978, Roberts was "a pretty politically radical guy," said a colleague there. "I would definitely have put him into the left area, and this was in the civil rights division, in which most people were pretty liberal."

Roberts won some latter-day slavery cases, arising from abuses in North Carolina migrant labor camps. "Crew leaders would go into Southern towns, skid row, get 'em into a truck, and load 'em up with liquor," explained another colleague at Justice. "They'd wake up the next morning in the fields, and when they tried to leave they'd be beaten."

In 1981 came his best-known trial. Joseph Paul Franklin, who would also be accused of shooting civil rights leader Vernon Jordan, was charged with killing two black men he saw jogging in Salt Lake City with white female friends. Franklin said the men deserved to die for "race-mixing."

With a local assistant U.S. attorney, Roberts won a conviction. At sentencing, the 200-pound defendant could not restrain his rage. Franklin stood up, called Roberts a "trained ape," and charged the prosecution table, upending a water pitcher but not quite reaching Roberts before deputy U.S. marshals wrestled him down.

"Roberts made light of it," recalled Dan Rinzell, a colleague who would be unable later to win a conviction in the Jordan case. "I think he was happy that it happened, in a sense, because if the judge had any doubt about how much {prison} time he was going to give {Franklin}, that resolved it."

After a brief stint at Covington & Burling, Roberts became an assistant U.S. attorney in the Southern District of New York. There he acquired the reputation of a legal scholar and meticulous investigator, and he gave up civil rights for white-collar crime.

"He's the envy of everybody in terms of organizing things," said Michele Hirshman, who worked with him in a satellite office in White Plains. "He would create these checklists that still get passed down through the generations here."

Roberts returned to Washington for love. When a long-distance courtship led to marriage to McCann, he joined the U.S. attorney's office here in November 1988.

Within a month he was assigned to the Barry case, just unfolding in Charles Lewis's room at the Ramada Inn.

Well past dark on Jan. 18, Retchin shared a quiet break with Barbara A. "Biz" Van Gelder, her best friend at the "Triple Nickel," as the 555 Fourth St. NW U.S. attorney's headquarters is sometimes called.

As they chatted, Retchin knew that a colleague, Daniel Bernstein, was at that very moment in Room 726 of the Vista Hotel, watching Marion Barry and Rasheeda Moore on closed-circuit monitors. But Van Gelder, a senior prosecutor herself, would learn nothing of that drama until she reached home and switched on a television set.

"The buzzer went off, she went to another room to take a call, and when she came back she said, 'I'm going to be busy,' " Van Gelder recalled. "I came up to her the next day. I couldn't believe it. 'You didn't even tell me!' "

Later that night, Retchin called another close friend, with whom she has shared a house for 14 years. The housemate spoke to a reporter on condition that her name be withheld.

"She said, 'Will you bring us some Chinese food?' " the woman said. "I said, 'Now? It's late.' She said, 'This is really important. There's nobody I can ask to do it. The mayor's just been arrested.' "

Retchin's work has demanded of her the strictest kind of immunity from the urge to gossip. Like a CIA spook or a deep-cover cop, the 38-year-old prosecutor has spent the last 18 months learning astonishing things about the mayor -- and been unable to tell a soul about them.

Even if she'd been tempted, it would have been hard to find time to discuss her work. Driven by Stephens's determination to bring the Barry investigation to a close, Retchin and Roberts have worked prodigious hours together, often remaining in their offices past midnight and returning by 8 a.m. for another day.

Like Roberts, Retchin came to the assignment by way of the Justice Department, but her experience was in the arid reaches of antitrust, which rewarded patience and mastery of detail. Retchin became an assistant U.S. attorney in 1984. "A superb lawyer," said Joseph diGenova, Stephens's predecessor, who hired her.

Retchin is "the kind of kid in grade school who did her homework every night," said defense lawyer Greta Van Susteren. "There are certain lawyers I'd rather not come up against. Judy's a lawyer I'd rather not have assigned to my case, because she's good."

A cool clinician in the courtroom, she has tried perhaps 100 cases with scarcely a change of demeanor. Unlike many colleagues, who tailored their presentations to each case, Retchin gave the same consistent performance: efficient, unflappable, relentless. "I have always described her as the Sergeant Joe Friday of the office," Van Gelder said. "Just the facts."

Those who know her warmer side say Retchin's intensity in the courtroom can be unsettling. "She's my sister, but she's also one of my best friends," said Nancy Retchin, 29, a New York artist, "and to see her up there is kind of frightening. I don't know, but I wouldn't want to be up against her."

In D.C. Superior Court, where she learned her criminal law, preparation became Retchin's signature.

"Her attention to detail was just exemplary," said Judge Henry H. Kennedy Jr. of the year she spent trying felony cases in front of him. "I always sensed that she was able to make her version of the facts believable because of the details she was able to elicit."

Retchin can look stiff against a performer like Mundy, Barry's lawyer. But she has beaten him before, and she has done it by marshaling her facts. A year ago she won a mandatory life sentence against Mundy's client, Michael Palmer, for running a "continuing criminal enterprise" in the District.

Mundy, a noted cross-examiner, complained to Judge Harold Greene at a bench conference that his job was like pulling weeds: For every witness he tried to discredit, Retchin sprouted another.

If there is any common criticism of Retchin's performance, it is for her nearly unpierceable cool. "She tends to be a very rigid, humorless person," said defense lawyer Allan M. Palmer, his remarks echoing comments from several others.

"I think that's men trying to put a label on a good female prosecutor," retorted Mark Shaffer, who represents government witness Charles Lewis. "Some men have a lot of trouble with women in an authoritative role."

Friends say the private Retchin is softer. "Judy's the sort of person who, if she notices somebody in a roomful of people ... who's a strange face, she will go up to her, introduce herself and try to make her feel included," said one friend.

Even so, Retchin reveals herself slowly. Few among her acquaintances profess to know her well.

A two-time undergraduate "athlete of the year" at George Washington University, where she played basketball, she continued to excel at sports at Catholic University Law School. Today she remains competitive on a tennis court.

Food, according to some friends, is among her passions. "She'll drive miles and miles and miles because she's heard somebody makes a good pizza," her housemate said. Her favorite pie of the moment comes from a pizzeria in Alexandria called Generous George's.

When Roberts is ready to start a cross-examination, he stands and assumes The Position: shoulders high, legs splayed, leaning into the lectern like a lineman. He is a tall man, and he makes the most of it.

Right arm forward, left arm cocked, he knows exactly what every question will elicit. Prosecutors, to prepare their case, have had grand jury subpoenas and "men with badges and guns asking questions," as defense lawyer Mark J. Rochon put it. Mundy and co-counsel Robert W. Mance, unable to compel testimony in advance, have sometimes had to fish in front of the jury.

Roberts has handled fewer major witnesses than his colleague, but he drew the biggest one of all -- Marion Barry -- only to lose his chance in the end. Few strategists expected the mayor to testify, but Roberts had to prepare for cross-examination as though he might. He spent many long nights in the office framing questions he would never ask.

One night in June, Roberts took a few hours off for personal business. With thousands of Washington residents, he lined up at the D.C. Convention Center to celebrate the first U.S. visit of Nelson Mandela. The emotion of the moment, for an old apartheid protester, must have been powerful and full of joy.

But what could Roberts have been thinking when Barry received a standing ovation from the crowd?

Like Roberts, Barry had his dashiki days, and he too made his name in civil rights. As the two men met each day in Courtroom 2, Roberts found himself in a potentially uncomfortable role.

"I don't envy the position he's in, being a black person prosecuting a popular black figure," said Claude Bailey, a special counsel with the D.C. Corporation Counsel who also counts himself a friend. "He's obviously susceptible to the criticism that he's just a quote-unquote tool of the people trying to drive the mayor out of office."

An unavoidable subtext of the trial, race pervades every analysis. During a brief recess in the second week, Lurma Rackley, the mayor's spokeswoman, complained that courtroom artists had darkened Roberts's complexion -- as if to stress that the mayor was being tried by a black man. "He's very light," Rackley said.

If Roberts is troubled by the subject, he shows no sign.

"I'm sure there are some members of the black community here in Washington, D.C., who will never, ever forgive him for what he's doing," said another friend. "But when I say {to Roberts}, 'I've been worrying about you,' it's, 'No problem, I'm fine.' "

Sarcasm in front of a jury is not Retchin's strong suit, and she must know it. Full of droll wit in bench conferences, she only twice attempted to use it in open court.

Detective James L. Stays -- confident, magnetic, 30 years on the force and damned proud of it -- presented a problem. After 11 years on Barry's security detail, he testified he had never seen a sign of drugs, not even at places and times in which the mayor is charged with possession.

Retchin thought she knew Stays inside out. She had examined a hundred officers over the years. Usually they were allies; Stays was not, and they both knew it.

Waving a government-issued ballpoint, she tried at one point to establish the detective's loyalty to the mayor. "He has as much control over you," she scoffed, "as I do over this pen; isn't that right, sir?"

Stays knew a rhetorical question when he heard one. "Do you want me to answer that?" he shot back.

The moment belonged to Stays. But Stays remained an obstacle, and Retchin knows what to do with obstacles.

Within minutes of the confrontation, she revealed at a bench conference that she had evidence of drug use by Stays. Last Friday she got a chance to prove it, placing DeLois Mise on the witness stand to testify to her cocaine use with Stays. D.C. police promptly removed him from the mayor's security detail.

A long trial, most criminal lawyers say, magnifies the role of the opposing lawyers' style and personal flair. With greater exposure to the jury, according to lawyer Kenneth Robinson, "the better courtroom personality will rise to the top like cream."

Few strategists in Washington doubt that Mundy is the magnetic personality of this case. When closing arguments are heard today, he will be drawing hard on the trust he has built with the jury.

Prosecutors will not say who will handle the government's closing. They go so far as to claim that it does not matter.

In the end, said William D. Pease, a former supervisor in the office, "It's not {Roberts and} Judy Retchin against Ken Mundy. It's the evidence against Ken Mundy."