JUST WHEN YOU THOUGHT IT WAS SAFE TO GO BACK INTO THE GREENHOUSE, . . .

Rick Moranis raised the ax high over his head. Screwing up his face in revulsion, he swung mightily at the body lying at his feet. Chop! Bits of bone and blood flew all over the stage.

Slowly he disengaged the ax from the carcass and hefted it up again. Chop! Splat! As he severed the body limb from limb, his newspaper apron became progressively more disgusting.

It was food preparation time at Mushnik’s Florist Shop on Skid Row. The insatiable Audrey Two, a very greedy plant indeed, was about to be served a particularly sweet delicacy: chunks of Steve Martin. Later, the man-eating bloom would feast on mouthfuls of Mushnik himself (as played by Vincent Gardenia). Eventually her appetite would encompass all of North America.

A carnivorous plant doesn’t sound like a natural subject for musical comedy. And yet when “Little Shop of Horrors” opened Off-Broadway four years ago, the sight of a giant Venus flytrap cavorting on stage with a lissome young blonde in skintight clothes attracted more than just a few botanists. Vines dropping menacingly into the audience only increased the fun.

The play’s success encouraged film producer David Geffen and Warner Bros. to turn it into a film. This is the second screen outing for “Little Shop,” which started life in 1960 as a quickie Roger Corman movie. It developed a cult following at least partly because of the short but memorable portrayal of a pain freak by a neophyte actor named Jack Nicholson.

“That old film was terrible,” said Frank Oz, in the midst of directing the new one. “Yet I loved it. It was so cheesy, but that was the charm of it. It was fun.”

Oz, who created the voices and personalities for such characters as Miss Piggy, Cookie Monster and Yoda, has worked alongside Muppet creator Jim Henson since he was 19. He directed “The Muppets Take Manhattan” and co-directed “Dark Crystal.” By the time he photographs the last plant tendril in March, he and his crew expect to have spent $20 million.

“ ‘Little Shop’ is deceptive,” Oz observed. “It looks simple, but it’s not. If it’s played too campy, then the audience won’t care about the characters. But if it’s not campy enough, it becomes a drama and people will care too much. It’s a very difficult line to tread.

“I’m trying to keep it simple and intimate,” he added, “rather than ballooning it up into a musical ‘Gandhi.’ Of course, when you see the set on 007 (Pinewood Studio’s enormous James Bond stage), you say, ‘Oh, that’s where the money went.’ ”

Production designer Roy (“The Killing Fields”) Walker converted the huge sound stage into Skid Row. Here in the dilapidated basement of Mushnik’s Florist Shop, Seymour Krelborn (Moranis) nurtures an exotic plant he bought from a Chinese mandarin. Because it is so pretty and delicate, he names the plant after his secret love, Audrey, his fellow shop assistant (Ellen Greene, who originally created Audrey for the stage).

Audrey is under the power of local dentist Orin Scrivello (Steve Martin), a heavy-leather biker whose chief attraction to dentistry is his enjoyment of inflicting pain. Seymour doesn’t know how to deal with his competitor until he realizes just what nourishment Audrey Two requires.

Before Orin meets his destiny in the jaws of Audrey Two, he finds a patient who appreciates his approach to tooth care. Bill Murray agreed to play the pain-loving patient (Nicholson in the 1960 version) on the condition that he be uncredited. Also appearing in cameo roles are John Candy, Christopher Guest and Paul Dooley. Levi Stubbs, lead singer of the Four Tops, provides Audrey Two’s voice.

England has been particularly frigid and 007 is not totally enclosed. At one point the heat was off for five days--and the stage was only slightly warmer than the parking lot. Only Moranis, who spent several hours chopping up bodies, was sweating. Everyone else was bundled up in ski parkas, scarfs and gloves.

Moranis, whose curly hair, glasses and baby face seem just right for the timid florist’s assistant, bragged about his “black thumb. I had a dead plant gallery in my apartment once. I’m good at growing mold and fungus.”

Instead of studying horticulture for the part, he took singing classes. “I’ve gone through a few vocal teachers,” he shuddered. “I remember one who took me down to her basement, put me on the most shmutzik rug I ever saw and told me to make duck sounds for 20 minutes.”

Except for “Strange Brew,” which made Moranis a household name in his native Canada, “Little Shop” is his first above-the-title part. “After I left ‘SCTV’ and made a couple projects that didn’t work out well (‘Streets of Fire,’ ‘The Wild Life’), I realized I was missing a good, healthy working environment,” he said. “I’m not interested in working for autocratic directors. Frank is like a co-performer. I feel like I’ve known him for years.

“My role in ‘Ghostbusters’ was too small for me to feel part of things. Ivan Reitman is a really efficient producer/director. ‘Ghostbusters’ was very economically and creatively made. Harold (Ramis), Danny (Aykroyd), Bill (Murray) and Ivan knew each other’s style. They would discuss how to get the funny line, get it and then move on.”

Moranis examined his bandaged fingers (he was feeding Audrey Two his own blood before he moved on to corpses). “I still don’t think of myself as an actor. I’m a writer. The first thing I did on ‘Ghostbusters’ was write all my own material. ‘Little Shop of Horrors’ is really new terrain for me. I’m trying to breathe life into a character whose lines are good already.”

Ellen Greene’s biggest challenge has been to prevent her larger-than-life stage portrayal of Audrey from becoming too big on screen. “I tried to find films to study that could possibly help,” she said. “But there weren’t any. ‘The Rocky Horror Picture Show’ is jaded and campy. ‘Little Shop’ is about innocence. The movie that comes closest is ‘Breakfast at Tiffany’s.’ ”

A one-time New York cabaret and gay bar singer, Greene made her film debut in “Next Stop, Greenwich Village” in 1976. Six years later she appeared in “I’m Dancing as Fast as I Can.” In between, she starred in numerous New York theater productions including the original “Little Shop of Horrors,” which she took to Los Angeles and London.

Greene’s stage visualization of Audrey was strong: “To begin with, Audrey wasn’t blond, sexy or round,” she recalled about the initial Howard Ashman book and lyrics. “I envisioned her as someone ripe enough to fall off a tree. She does have the lowest dresses in the world. She has two cleavages, one in front and one in back. I could tell the costumes were right because of the pain they caused.”

Lyle Conway has been working on the plants for nine months. A one-time Chicago toy designer and now a leader in the field of animatronics, Conway first worked with Oz on “The Muppet Show” and has since created characters for several films.

He has a crew of 35. “This has to be the most difficult project I’ve done,” he said. “We’ve got seven plant sizes, numerous specials and a lot of vine movement. Frank really insists on these plants doing everything. They have to sing, dance, boogie and eat people. He’s not settling for anything short of photosynthesis.”

Oz has spent 30 of his 40 years bringing inanimate objects to life. “I know how hellish it’s going to be,” he says of the plant photography. “The actors are going to have to be very patient.”

Although the plants are under wraps, Conway gives hints: “The small versions are very precious and feminine. I was working from a caricature of Ellen’s lips. At 4 feet, Audrey Two is pushing her luck a lot. She really knows what she wants. At 5 feet, she goes and gets it.

“The biggest version is pretty brutal. She’s 12-and-a-half feet tall, has 15,000 leaves and vine-like arms that do a lot of grabbing, touching and dialing telephones. She has a definite shark-like look--a big, open mouth with teeth in it. When Ellen goes swimming, a leaf follows her like a shark fin protruding.

“This must be the first musical comedy where the entire cast gets eaten. However, I’ve seen a couple where the entire cast should have gotten eaten.”