Everyone knew it was coming, but the jurors had to wait until well after lunch to see the FBI's Vista Hotel videotape yesterday. Instead of simply screening it, prosecutors in the mayor's drug and perjury trial took the unusual step of previewing its contents in testimony that lasted all morning.

The decision to construe the tape before displaying it was as good an indication as any that the government's most spectacular piece of evidence against D.C. Mayor Marion Barry is also the most subject to interpretation. Both sides struggled yesterday to resolve any ambiguity in their favor.

U.S. District Judge Thomas Penfield Jackson said at a bench conference that he had expected the government to let the tape "speak for itself." But the jurors' subjective reactions were far too important for either side to leave to chance.

Assistant U.S. Attorney Judith E. Retchin used two hours of testimony by Hazel Diane "Rasheeda" Moore to provide a moral and factual context for a tape that has been called murky in both respects. Defense attorney R. Kenneth Mundy, whose opportunities yesterday were more limited, used so-called "speaking objections" to plant seeds of doubt about Moore's preview to the jury. And Mundy announced his intention to play the tape twice in cross-examination -- the second time "in stop frame," to emphasize points favorable to the defense.

Moore and the tape in which she stars are only peripherally relevant to the three felony charges of perjury that raise the worst risk of prison for the mayor. Moore had no testimony at all on whether Barry ever gave cocaine to Charles Lewis or received it from him, and her testimony about a Virgin Islands boat trip -- in which, unlike Lewis, she said she saw no cocaine -- offered more evidence for the defense than the prosecution.

But without Moore and the tape, it is likely that there would be no case against the mayor.

The tape itself is evidence of one instance of cocaine possession. Moore's testimony has spoken to many others. And Moore has offered by far the best evidence to date that Barry, as the government charges, entered a six-year conspiracy to possess cocaine.

More important, perhaps, is that the model-turned-government witness has always lurked near the emotional heart of the case.

No theme emerged more strongly in two weeks of jury selection than the discomfort with undercover stings and the hostility toward Moore felt by many potential jurors -- including several who made the final cut.

There is no escaping the fact, of course, that Barry is right there in black and white smoking cocaine. Legal experts outside the courtroom, some of whom watched the tape on television yesterday, said that image would be devastating to the defense.

Though most thought "good lawyering" by Mundy could find pieces of the picture to exploit, many agreed with Greta Van Susteren that he would "need a magic wand" to make the tape look like evidence in his favor.

"The problem remains," she said. "The government has so much more on the tape than the defense does. Mundy will put up a good battle, but they have better ammunition."

Georgetown University law professor Paul Rothstein, who watched from inside the courtroom, believed the tape could backfire on the prosecution, giving Mundy "his only arrow to shoot, that there's government misconduct here." Though misconduct is not an issue for the jury -- and the jurors will be instructed firmly not to consider it -- both sides have privately acknowledged that the jurors may disregard that instruction.

Retchin, in previewing the tape, took several steps to insulate Moore from that kind of criticism in the jury room. She elicited testimony from her witness that Moore decided to lure Barry to the hotel room in part out of concern for the mayor's health; that Barry had been feigning when he professed not to know how to smoke the crack; and that Moore had done nothing at all to pressure Barry into smoking it.

Mundy, whose time had not yet come for cross-examination, sounded out his objections from his chair instead of approaching the bench as the judge had requested.

"She is not a medical doctor," Mundy said, objecting to Moore's portrayal of crack use as growing from a disease.

"Now she's a minister," he exclaimed, in mock exasperation, when Moore began quoting scripture to make a point.

In effect, he told the jury to be skeptical of the Moore and the spin she sought to place on the videotape.

The tape itself, the experts said, offered evidence that each side could exploit.

For the defense, they said, potential themes included: the acknowledgment by FBI agents of the risks to Barry's health of cocaine ingestion; what Rothstein called the "sleazy cast" to the tape and its "peeping tom" quality; and what lawyer Bernard S. Grimm called the image of the defendant as a vulnerable, middle-aged man with "foibles and frailties" like everyone else.

For the prosecution, experts said, nothing is more important than the stark reality of the mayor's drug use.

Several lawyers said that reality would blunt Mundy's attempt to show that the mayor was entrapped. That defense requires "an unwilling participant," not merely a "reluctant one," said defense lawyer Donald Bucklin.

All the tape seems to show, he said, is Moore and Barry "arguing about who is going to do it {smoke crack} first, not whether they were going to do it. That is not an entrapment defense."