Those who have been there before, in the searing spotlight before a congressional committee as Bert Lance is now, tell us it's not easy. Looking at some witnesses sweat through the ordeal, that's obvious. Others make it look easy. Look at Lance. He seems confident. Remember the self-assured John Dean? But behind the facade . . .

"I heard my heart pounding hard . . . I was worried that I would have to go to the bathroom every five minutes," Dean wrote in his book, "Blind Ambition." He made an opening friendly remark. "But there was no reaction, not even an understanding look."

The nation watched that historic session, just as the current Lance hearings are attracting a large nationwide audience. But the Capitol Hill process of testifying goes on day-to-day, a peculiarly Washington business.

There are 22 House committees, 15 Senate committees, and more than 200 subcommittees althogether, and they all hold hearings. Who doesn't get to testify? Among those who do it are the famous, the infamous, personalities wanted by committees to hype their interest, the unwanted who still manage to get their say in for the record, pros who do it for a living, amateurs making one-shot appearances.

In a super-scrutinized case like Lance's, where the stakes are very high, it's time to call on the pros.

So there sits Clark Clifford serving as Lance's attorney - the impeccably groomed, silver-haired, calm, gracious, skilled Clifford, a former Secretary of Defense who knows what maneuvers are called for. It's almost as if an aura of respectability emanates from him, cloaking his client.

"Clifford accomplished things in Washington about which other attorneys dare only dream," writes Joseph C. Goulden in his book, "The Super-lawyers." He refers to a Clifford "mystique."

"You saw that statement that Lance gave, it look easy," a Washington lawyer said after the budget director's opening remarks on Thursday. "But hundreds of hours went into that. Poring over documents. Losts of sleepless nights. Clifford did a great job . . . I wonder who did the poring over the documents?"

Far more commonly, of course, testimony on the Hill deals with legislation, a process that often attracts less attention and is thus less nerve-racking for a witness, but still has its frustrations. Witnesses, for example, sometimes wonder if they're even being heard. What they see on the panel in front of them can sometimes, at the least, be highly distracting.

"Staff mill around, whisper in the ears of the congressmen," says James Banks, who testifies regularly on environmental issues for the Natural Resources Defense Council. "Sometimes there's a good deal of reading by the congressmen, talking among themselves."

Worse, sometimes few committee members are present, maybe only the chairman. Sometimes a witness gets little chance to say anything despite a good deal of time preparing and considerable travel expense.

"In 1974, there were several witnesses who had come from as far as Montana," says John McCormick of the Environmental Policy Center, who has testified on strip-mining legislation. "There were six speakers and they were told they had a total of three minutes. They were given a half-minute each. They had spent over $400 to get there. We convinced the committee to come back in the afternoon."

In getting a committee's attention, the initiated say, a witness must be careful not to put anyone to sleep by reading a long statement.Better to summarize, adding a fresh anecdote or two. Visual aids help, too; in fact, industry representatives often come equipped with elaborate charts and other materials.

But when it comes to theatrics, one's success may depend on the particular panel. Earlier this year, S. John Byington, chairman of the Consumer Product Safety Commission, met with an abrupt setback while testifying before a House subcommittee on Tris children's sleepwear.

When one congressman asked him whether he would allow his own children to sleep in washed, Tris-treated nightwater, Byington tried to bring his two daughters to testify. But the chairman, Rep. John E. Moss (D-Calif.), rapped his gavel repeatedly, cut Byington short and accused him of "increadily bad taste, poor judgement."

No matter how it goes, a Washington lawyer said, "stay cool, don't go into the big tirade." Sometimes it helps to smile. Even Melvin Laird, who has been hailed as "a master of the American political process," reportedly has received that advice.

It helps to say something unexpected, add some new approach, which Walter Fauntroy is said to have a knack for when it comes to testifying on District voter representation in Congress.

Appearances count for a lot, in most cases. As one of Dean's lawyers told him. "Put on your Jimmy Stewart clothes, boy. You're going to the Senate." It doesn't pay to show up in jeans. Dean, making precise preparations, discareded contact lenses, which caused him to blink, in favor of horn-rimmed glasses, which caused him to blink, in favor of horn-rimmed glasses, which he believed added a more distinguished touch. And he got a haircut.

None of this will mater much, though, if a witness is not prepared. Just as Lance's testifying team has done its homework, a wise witness, or his advisers, will know what to expect. "Scouting" plays a part.

"i've sent an intern to watch the preliminary rounds," says the NRDC's Banks, who will testify before a House committee Monday on water pollution. But he wouldn't reveal any information he'd learned about the committee members. 'I don't want to antagonize anyone,' he said.