Whitney North Seymour Jr., the New York lawyer and the man who would prosecute former White House aide Michael K. Deaver, was telling a federal appeals court this past June how difficult it was to pick a jury in Washington.

"These are reasonably unlettered people, mostly verbal," Seymour said. "To ask them to fill out a questionnaire to state their reason would almost be a joke."

It was a moment that Michael K. Deaver's lawyers were to remember with glee, a signal that their opponent in what was to be one of the most important criminal trials in the capital for a decade was a Washington novice, unfamiliar with the city's courts and especially its jurors.

Yet when the Deaver jury filed back into Courtroom No. 2 in the U.S. Courthouse on Wednesday afternoon, it had sided solidly with Seymour, finding the former White House deputy chief of staff guilty of three counts of perjury.

The verdict doubly shocked many in Washington's legal community. Seymour, 64, the Yale-educated lawyer from Park Avenue, had convicted a man who once was one of the most powerful political figures in Washington and had done so by outmaneuvering one of the city's premier criminal lawyers.

Herbert J. (Jack) Miller Jr., 63, the gregarious George Washington University-educated lawyer who led the Deaver defense team, tersely acknowledged that he may have underestimated the impact of Seymour's case and said the jury's verdict suggested he may have erred in his surprising move not to put on a defense.

To Peter Megargee Brown, Seymour's Manhattan law partner, it was an understandable mistake. "I've seen it happen a lot. He {Seymour} comes on as someone who is honest, open, and people say, 'Come on, I'll take him by half.' "

Indeed, by many Washington standards, Seymour was not likely to be viewed as one of the powerful. He is a former federal prosecutor, but in his one major political outing, a bid for the U.S. Senate from New York in 1982, he was crushed in a Republican primary.

Someone who celebrates the Fourth of July by reading passages from the Declaration of Independence with his family, who runs a small literary press, who is cochairman of a group opposed to political action committees and who had not prosecuted a criminal case since 1973 did not seem a heavyweight by Washington standards.

Not when he was pitted against Miller, who had been called to defend Richard M. Nixon in his final days as president, who had served as the self-professed "token Republican" of Robert F. Kennedy's Justice Department and who was considered one of the most savvy courtroom advocates in the city.

The contrasts were striking. The prosecutor was urbane, pedantic, aloof, always erect and always attired in blue, button-down shirts.

Minnesota native Miller was folksy, a bit rumpled and potbellied, with legs bowed by the two heavy briefcases that he toted in and out of the courtroom.

Miller and members of his firm have said little publicly since the verdict, but a review of the trial and interviews with lawyers indicate that Deaver's defense team may have been lulled by its perceptions of Seymour and his often-condescending courtroom style into questioning whether he was capable of mounting a credible case. The defense and prosecution frequently squared off, with Miller often accusing Seymour of "outrageous" conduct for a prosecutor.

In the end, it was Miller who was admonished by U.S. District Court Judge Thomas Penfield Jackson for personally vouching to the jury for Deaver's honor, a breach of legal etiquette that may have shown how little evidence the Deaver defense team had placed on the record to support its view of the case.

"I don't know if it's fair to say we underestimated Seymour," Randall J. Turk, one of Miller's associates, said yesterday. "But, certainly our view, at the close of the prosecution, was that it was a weak case."

Although Miller and his two Georgetown University-educated colleagues, Turk, 37, and Stephen L. Braga, 32, had made a point of painfully dissecting the testimony of some witnesses, the defense had been kind to others. When, for example, former White House lawyer Frederick F. Fielding told how he cautioned Deaver against lobbying former White House colleagues, Miller's questioning did not stress that Fielding's job was to worry more about President Reagan's image than Deaver's professed right to make the contacts.

Miller's greatest weakness may have been his final argument, which seemed to ramble and lacked the oratorical flourish and substance that Seymour's closing argument and rebuttal gave to his case. Miller essentially urged the jury to cast off the six weeks of testimony and to accept his belief that the Deaver case was little more than the personal product of a vengeful prosecutor.

Seymour only sought to indict the former Reagan aide for perjury after he was frustrated to find that he could not prosecute Deaver for violations of the Ethics in Government Act, Miller contended. That is the Watergate-era legislation that restricts lobbying by former senior government officials, and from the outset of the Deaver investigation in 1986, it was the law that he was suspected of violating.

Seymour had saved his biggest surprise for the end. He stayed in the courtroom until 11 the night before his final summation, working with FBI agent Eugene A. Rugala on just when to flash copies of memos on a big screen before the jury.

The result was the Whitney North Seymour slide show, a travelogue that zipped the jury through hundreds of pages of documents that 52 witnesses had described over the previous six weeks. The jury was nodding with approval as Seymour ticked off the ways he charged Deaver had traded on his friendship with the Reagans.

It was only one of the tactics that Seymour used to keep Miller off guard. Working out of a spartan office on the 6th floor of the U.S. Courthouse, Seymour assembled a staff that sometimes included as many as 10 lawyers who pored over Deaver's congressional and grand jury testimony and helped in composing the 18-page, five-count indictment that alleged Deaver was lying when he said repeatedly that he could not recall various contacts or that he did not contact administration officials on behalf of his clients. The charges were worded so that Seymour had multiple ways of proving Deaver lied on the various counts. If he could prove even one contact and had supporting documents, Seymour could win on some counts. Miller argued that the tactic crippled the defense.

With the help of two FBI agents loaned by then FBI Director William H. Webster and a team of lawyers, most of whom had worked as federal prosecutors in New York's Southern District, Seymour built a case that depended almost as much on documents as on witnesses. "One thing any law enforcement officer will tell you is that documents are wonderful witnesses," Seymour said. "Paper does not lose its memory or change its memory."

Seymour virtually moved to Washington soon after his appointment in May 1986. "He knew the case inside out, backward and forwards," said Marc J. Gottridge, one of his three top lawyers.

Seymour said he applied three rules to the case:

"Number 1. Your client is the government of the United States. You owe to them to be honest and straight with the facts. . . . "

"Number 2. You prepare, prepare, prepare."

"Number 3. You never assume a damn thing."

There were more than 20,000 pages of documents from Deaver's firm alone to digest and even more from the White House and the agencies Deaver was suspected of visiting, plus telephone bills to analyze.

But for the prosecutors, the key was to assemble a theory of what went on. "If you don't understand how the pieces fit together, you don't have a case," Seymour said.

Hundreds of names of Deaver's contacts were fed into a computer. If any witness interviewed by the FBI agents, the lawyers or the grand jury mentioned a name, it was logged into the machine, giving Seymour a vast data base on which to check and double-check any statement.

It was well after Deaver's June 20, 1986, grand jury testimony that Seymour said he and his staff began to believe "that Deaver hadn't been truthful" when he said he could not recall certain contacts he made for his clients, some of whom paid as much as $475,000 for his help.

Under perjury laws, a prosecutor must prove that the events the defendant lied about actually happened. To prove perjury, Seymour's staff had to prove what contacts actually were made. Seymour said that made the case more difficult than any he had ever tried.

"There are two principal reasons: the reluctance of the witnesses and the complexity of human motivation," he said. In short, the witnesses were mostly political and none of them wanted to rat on one of the president's longtime friends.

Throughout it all, Seymour remained isolated, avoiding social events and working late in his courthouse office to develop his theory of the case. "I was impervious to the cocktail party chatter. I really did want to be independent."

He would start the day with a 7:15 a.m. working breakfast with his key assistants. They were Gottridge, 34, a Harvard-educated former assistant prosecutor from New York; George F. Hritz, 39, a Columbia Law graduate from New York, and Walter P. Loughlin, 37, a Yale-educated lawyer and former federal prosecutor from New York, who was an associate professor of law at Rutgers. Hritz alone had Washington experience, having worked on the Senate Select Committee on Ethics that investigated the lobbying by South Korean Tongsun Park in 1977-78.

Seymour shunned the Justice Department, although he talked to both U.S. Attorney Rudolph W. Giuliani in Manhattan and Joseph E. DiGenova in Washington when he set up his office. He refused to accept a backup Justice Department appointment when the law under which he was appointed was challenged, and he drew his own office seal, featuring an eagle whose head is defiantly turned the opposite of the one on the Justice Department seal.

To Seymour there was little doubt that he could win the case.

"It doesn't do any good to bring cases and then lose them, especially cases as visible as this," he said.