Actress Blair Brown is a Jackie Kennedy dress-alike in her pink Chanel wool suit and matching pillbox hat, waving mechanically from the back seat of an open Lincoln Continental. Cheering spectators wave signs that read "We Love You, Jackie" and "Welcome to Dallas."

Suddenly, a gunshot rings out. As the crowd ducks for cover, Brown turns to watch her seatmate, Martin Sheen, grasping for his throat. Another shot. Sheen lunges backward and Brown, her suit now splattered with red dye and her mouth agape in horror, crawls onto the trunk . . .

It was a harrowing moment here last week when a huge crew from Britain's Central Independent Television and a crowd of several hundred extras staged the first filmed reenactment of a scene frozen in the world's memory--John F. Kennedy's assassination on the streets of Dallas, Nov. 22, 1963.

"It was horrifying," said Brown later. "From an actor's point of view, it's so much harder to do when you know it actually happened to someone else."

"You keep thinking, 'What if? What if? What if?' " added a subdued Sheen. "What if they hadn't turned down Elm Street? What if he hadn't gone to Dallas?

"It's been very sad here for the past few days," said Sheen, who insisted he turned down the offer to play Kennedy four times before acquiescing. "But you have to put your feelings aside and get on with the job."

A strange job it has been, too, both for the actors and the residents of this old Confederate capital, who have enthusiastically welcomed the production, convinced that they finally have landed big-time show business. "Kennedy," a seven-hour re-creation of JFK's presidency, has been purchased by NBC and scheduled to run in the United States and in Britain this November during the week of the 20th anniversary of the assassination.

"This is going to be bigger than 'Roots,' bigger than the Super Bowl," said one awe-struck college student who signed on as an assistant.

Whether or not the show lives up to such high expectations, the production team has gone to extraordinary lengths to replicate the look of the event and the era. Sheen and Brown watched the famous film of the assassination made by Abraham Zapruder three times to get the head reactions right. For the set, producer Andrew Brown and director Jim Goddard picked a deserted industrial street less than a mile from the state capitol. With the help of city officials, they knocked down a couple telephone poles and installed Dallas street signs under an overpass.

Extras--advertised for in local newspapers and on the radio--were under strict dress requirements: narrow ties, dark pants and fedoras for the men, scarves and long skirts for the women. Long hair, blue jeans, T-shirts and digital watches were banned from the set.

When the presidential motorcade began its first of an endless series of cruises down the block, extra Cindy Cull got chills down her spine. Twenty years ago, Cull had been in Dallas with her father watching the motorcade, just a few blocks from Dealey Plaza. "This brings it all back," said Cull, a Richmond warehouse manager.

For all the realism, director Jim Goddard insisted that this was not an exact duplication of the assassination since the whole sequence--about five minutes long--is being shown from Jackie's point of view. Thus, no Texas School Book Depository, no Oswald, no shots of mysterious characters on a grassy knoll. Also, no view of Kennedy's head being blown off. "I'm not making a story about what actually happened," said Goddard. "I'm making a story about what she Jackie thought happened. It will be all very fragmented . . . The backgrounds are all out of focus."

Goddard and others associated with the production actually have gone out of their way to downplay the assassination scene, clearly sensitive to any suggestion that they are dwelling on the macabre. Some cities might be queasy about the idea of being chosen as a place that bears even a passing resemblance to the Dallas of 1963. But not Richmond.

"We see it as a boost for the local economy and something that will improve the image of Richmond and give us the widest exposure," says Gene Winter, a city economic development official, who notes that the film crew will spend about $2 million during its seven-week stay here.

Producer Brown says the city was chosen because its skyline "had nothing to distinguish it"--not unlike Dallas in the '60s. The city also has a large number of old government buildings and architectural landmarks that have served as props for other events from the Kennedy era.

The state capitol steps became the site of Kennedy's Inauguration, the uppercrust Commonwealth Club was used for the Inaugural Ball and an old reconverted church on Broad Street served as the site of a civil rights riot in which white demonstrators, after surrounding a church where Martin Luther King Jr. was preaching, set fire to a car.

For that scene, scores of extras were hired as Alabama rednecks to shout taunts and hurl rocks at the black churchgoers. Meanwhile, city officials had to close off several blocks of the main thoroughfare, hose down the street and have a fire truck on the scene in case things got out of hand. Remarkably, says Winter, no one complained. "I think most people find it very interesting," he said. "I think they're getting into it."

There was no scene, however, that carried the emotional wallop of the final one. For more than eight hours on Thursday, under a bright summer sun, the mock presidential motorcade drove up and down the street, with Brown and Sheen smiling and waving to the crowd. The extras dutifully waved and shouted back, although many remarked at how eerie it all seemed, so close did it jibe with memories of that day in Dallas.

When the time came for the assassination scene, a kind of hush fell over the set. Three times in a row, director Goddard filmed the climactic gun shots, while a sound man ran alongside the Lincoln, firing blanks into the air. Getting the head reactions right was tricky, Goddard explained, because even after Sheen grabs for his throat, Brown's response has to be casual. "You have to remember that Jackie originally thought a motorcycle had backfired and he Kennedy was playing a joke," he said.

It was only after the final gun shot--the shot that never will be shown on the air--that Brown realizes what actually has happened. It is then that she begins her chilling crawl onto the back of the car. It was a reaction that appeared inexplicable to most witnesses until Jackie Kennedy's testimony to the Warren Commission: She was attempting to retrieve a portion of her husband's skull.

When it was all over, a somber Brown stepped out of the Lincoln and Sheen put his arm around her. "It's rather depressing," said director Goddard. "It's not the happiest scene in the world to film."