When Assistant U.S. Attorney Judith E. Retchin sat down to her fruit salad yesterday, she knew exactly what she faced in the courtroom after lunch. Her problem was Walter W. Bracey, the mayor's chauffeur, who had just finished testimony for the defense. If the jurors believed what he told them, Bracey could provide a pretty good alibi for a drug possession charge against his boss.

Retchin returned to court with a textbook cross-examination. When she sat down 40 minutes later, the nearly uniform view of the media was that Bracey's candor or memory -- or both -- had been badly discredited. "There's nothing left of him," said Richard N. Seaman, a lawyer who is watching the trial as consultant to WTOP news.

But a certain squirming in the jury box, and the comments of some spectators at the trial, suggested another perception of the exchange. Retchin, it seemed to some eyes, had done little more than ambush a man who did not have the courtroom savvy to fend her off.

There were more important moments yesterday in the drug and perjury trial of D.C. Mayor Marion Barry, but none more revealing. Like every fact at issue, the meaning of Bracey's testimony will be for the jury to decide. The disparity of courtroom reactions is a reminder that the decision will be deeply subjective.

It is an article of faith among criminal lawyers that most jurors make up their minds long before the close of testimony. Defense witnesses, in this view, are intended less to win converts than to gird them for battle in the jury room.

"By the time you're getting to defense witnesses," said Mark J. Rochon, a former chief trial lawyer for the Public Defender Service, "people will hear and see what they want to hear and see."

Bracey's testimony yesterday capped a two-day attempt by defense lawyers R. Kenneth Mundy and Robert W. Mance to provide an alibi for one of 14 counts in Barry's indictment. That count depends on testimony by acknowledged drug dealer Lydia Reid Pearson that she sold Barry three bags of crack shortly after 10 a.m. on Sept. 7, 1988, at the mayor's Frank D. Reeves Municipal Center office.

"This is the most specific count they've got," Mance said in a hallway interview. "If we can show doubt on this one, we can argue that all of them have doubt."

On Monday, two witnesses testified that Barry attended a meeting at D.C. fire department headquarters on Vermont Avenue NW some time after 10 a.m. on Sept. 7. Yesterday, two witnesses testified that Barry attended a ribbon-cutting for a Safeway earlier that morning.

Bracey was the alibi's cement. The mayor's 12-year chauffeur testified that he drove Barry straight from the Safeway to the fire department, with no stop at the Reeves Center or anywhere else.

Retchin, avoiding any trace of sarcasm, asked Bracey about his schedule from Sept. 6. "I can't remember," Bracey replied. Then she asked about Sept. 8. "I can't recall," he said.

"But you do recall . . . Sept. 7," Retchin asked.

"That's correct," Bracey replied, arms braced across his chest and voice growing gravelly at the challenge.

Hastily prepared for testimony by the defense, Bracey apparently was under the impression that it would not be proper to acknowledge that he had had his memory refreshed in advance with a copy of the mayor's schedule. Retchin helped foster that impression by asking the question this way: "Who did you talk with before you testified here today to jog your memory?"

No one, Bracey replied. He simply remembered. But Bracey, a man of simple language and stubborn demeanor, could not or would not remember anything else. Retchin made him repeat that assertion dozens of times in questions marked occasionally by laughter from the galleries.

"Out of all the dates that you have been with Mr. Barry, that is the only day that you remember. Is that right?" Retchin asked finally.

"That's correct," he said.

There was no dissent in the media rows from the view that Retchin had "blown {Bracey} away," as Seaman put it. But the view from the back of the room, where the general public sits, was rather different.

"I don't think it made him look bad," said spectator Walter Thomas. "I think it made her look bad, by pressing it as long as she pressed it. I don't think she made a point, other than the fact that she ran into a recalcitrant witness."

The 18 jurors and alternates showed no clear reaction but discomfort. Whether they were recalling their own experiences on the witness stand during jury selection could not be divined.