Lady Liberty is stalking up Fifth Avenue, and boy, is she big. As big as, say, the Stay-Puft Marshmallow Man. "Bigger than Stay-Puft!" hisses a publicist protectively. It's a frigid night near Manhattan's Grand Army Plaza, and the same folks who brought you the megahit ectoplasmic comedy Ghostbusters are filming the piece de resistance FX scene of their sequel. Hundreds of bemittened extras search the skyline expectantly for Lady Liberty to appear looming over Trump Tower, ready to save the Free World from the River of Slime biding its time in the maze of New York's sewer system. Of course, you can't really see Lady Liberty. Two cameras from Industrial Light & Magic film an empty Fifth avenue at 2:00 a.m. while the extras sing their lungs out to the vacant air. Later, when they finish rotoscoping, animation, matte work and other trickery, it will look as if these 460 extras really saw something that night.

It's been five years since Bill Murray, Dan Aykroyd and Harold Ramis first tried to save Sigourney Weaver from haunted kitchen appliances. The original cast has returned, as have producer/director Ivan Reitman and executive producer Michael C. Gross. "This story picks up after the last story," offers Gross at the Lady Liberty location, the last one before moving the tight 67-day shoot to Los Angeles for interiors. "Sigorney Weaver's thing with Bill Murray has gone sour. She goes off and has a baby with someone else." But that marriage had hit the skids, too, and now Weaver, a single mom working as an art restorer at the Metropolitan Museum, finds that both baby and stroller have gone for a supernatural ride at the corner of First Avenue and 77th Street. Who's she gonna call?

The Ghostbusters have split up to pursue separate careers, partially at the insistence of the City of New York, which revoked their license to bust ghosts after the boys tore up the West Side at the end of Ghostbusters. Dr. Peter Venkman (Murray) is now the host of a cable TV show on psychic phenomena. Dr. Ray Stantz (Aykroyd) and Winston Zeddmore (Ernie Hudson) are entertaining at children's parties. Dr. Egon Spengler (Ramis) is doing research at Columbia University. Rick Moranis, who played Weaver's erstwhile nerd neighbor, has gone to night school and become a lawyer. Nevertheless, who can resist a 6-foot damsel in distress? The Ghostbusters reteam for their biggest unearthy assignment, and Murray and Weaver rekindle those extrasensory perceptions they once felt for each other.

This time, the key to the evil forces behind the subterranean River of Slime resides in a painting, "Vigo the Carpathian," that resides in the Metropolitian (the old Customs House in Lower Manhattan doubles as that august establishment). Vigo, evidently, was "the nastiest person in history," and as the painting gradually comes to life, it unleashes evil. "The painting taps into the negative energy of the people in New York and gets meaner and meaner, until the River of Slime is ready to blow," enthuses Gross. "The painting's refinisher, played by Peter MacNichol, is commanded by Vigo to bring Sigourney's baby to him and help transform him from a painting to an entity and then take over the world." The script thus creates a "split nemesis," the gradually evolving painting and the River of Slime. But as far as technology goes, the slime was easier to recreate for the movies. "The painting has caused endless problems," admits Gross. "The technology is difficult to do. It requires a subtle movement. It can't work as just animation - that's too flat." Gross solved the problem by hiring the actors who stage an annual "Pageant of the Masters" in Laguna Beach, where they do lifesize reenactments of classical paintings.

Gross concurs that Ghostbusters II has had a long gestation for a sequel. "Initially, there were two problems: the availability of everybody, since we didn't think it would work unless we put back the key people, and did we really want to do a sequel? Would we want to make the same movie again? After three years, we said it was a good idea, but there was no one around. At a certain point, Ivan said, 'If we can do this, we have to plan it well over a year in advance and lock everyone's time up.' It's been great fun to see everyone together together again, like a high school reunion. It's a kick." Ghosbusters II was also held back by a late start when Reitman finished postproduction on the Arnold Schwarzenegger - Danny DeVito hit Twins. "The miracle was that Ivan was able to concentrate on postproduction on one movie and preproduction on another movie simultaneously," marvels Gross, who has worked in tandem with Reitman for eight years on such projects as Heavy Metal, Legal Eagles, Big Shot, Twins and, of course, the original Ghostbusters. Once begun, Gross reports the production was off to a good start, "like a bat out of hell. Ivan's very decisive. He knows what he wants and he goes for it."

Of course, they had a problem getting volunteers to come back later to get some New Year's Eve footage, so crucial to the movie's climatic scenes. "If I had a choice, I'd go to Beirut," frowns Gross. "You didn't see a lot of guys volunteering for shooting in Times Square on New Year's Eve." Gross claims the budget on the sequel is the same as the first: "About $25 to $30 million. The stars are deferring their salaries in the interests of keeping the film reasonable." As for the formula for a successful sequel, Gross professes ignorance. "I have no experience in sequels," he shrugs. "We don't know whether to give people a lot of the same material, because that's what they expect, or do that and make the same movie twice. How many surprises can you give them and keep it fresh? We wrestled with that all the way through. Most of these decisions are Ivan's. There was a stage where we had a major script point that would have taken so much away from the feeling of the group. It would have separated Bill a lot and taken the camaraderie away, so we changed it."

Meanwhile, Gross says the sequel will benefit from a new maturity. "Ivan's matured as a director, the stars matured as actors. Danny Aykroyd is more responsible; he tends to take care of business in a less boyish way." Actually, Aykroyd's interest in the supernatural is quite genuine; he is a member of the American Society for Psychical Research and is said to have told President George Bush at a screening of My Stepmother is an Alien to "call me if there's any trouble at the White House with the ghost of Mary Todd Lincoln." Meanwhile, there's the ghost of the first Ghostbusters to contend with. Known in Japan as Ghost-a-bust-a, in France as 100 Phantomes and in Germany as Ghosthunte, the original has ranked in around $200 million, a daunting figure. "The thing we've not done is sit there and say. 'Well, if we don't do this, we won't make X amount of dollars.' " insists Gross. "Merchandise is not on our minds when we do this. We don't want it to be the tail that wags the dog. Our first and biggest concern is how many days will it take to shoot the picture, how big is the crew, how much do we shoot? It's too expensive to shoot in New York, so we'll fake much of it in LA. We have a second unit crew shooting here every day that we're shooting, which we didn't do on the first movie."

Gross hopes the sequel will benefit from a whole generation of new fans. "Are we worried about it doing well?" he ponders. "We can't worry about it, we're just doing it. Will Ghostbusters II be funny? That's our main concern. The script is the funniest thing I ever read, and we're writing as we go." Now Gross points up to the cold empty air. "That's where she'll be," he says, referring to the larger-than-Stay-Puft strolling statue. She's not there yet, but Michael Gross has a vision of the future, and no one's gonna slime it up.