Michelle Johnson is puzzled, and you can't really blame her. Michelle, ripe to bursting at 18, is so new to Hollywood she just doesn't understand the uproar over her first film, Blame It on Rio. The critics didn't merely loathe it, they attacked it as "disgusting...dismaying...soft-core porn."

"People are taking this far too seriously," says Michelle, who spends much of the film with her ample breasts bared. "This film has no message. It's pure fantasy."

In this case, a prurient fantasy, designed to appeal to the Humbert Humberts of the world. Johnson plays a hot-to-trot 15-year-old whose love object is her father's best friend, married, middle-aged Michael Caine.

Despite the fact that she had never acted before, the statuesque 5'10" model was picked for the role from some 300 candidates. "I wanted a girl who was able to play extroverted," says Stanley Donen, the film's producer-director. "The character is very much like me, not terribly inhibited," Michelle says with a breathy giggle.

One major difference between Jennifer and Michelle, claims the actress, is Jennifer's limited sexual awareness. "She followed her heart instead of her head," says Michelle. "She really didn't realize her sexuality and how much control a woman can have over a man. She didn't seem to see what could happen down the road."

One thing Michelle sees down the road for herself is a hot-selling nude poster. It was all her idea, which doesn't seem to jibe with her claim that she found performing in the raw "very uncomfortable. I don't think anyone enjoys doing nude scenes," she says, "except maybe Richard Gere." As for the poster, which her managers claim is "almost an art piece," Michelle says, "it's implied nudity. You can't see any nudity at all." Except, maybe, in the mind's eye.

Johnson was born in Anchorage, Alaska, where her mother, Faye, was a nurse and her father, Don, was a furniture-store owner. After her parents separated, Michelle, then 4, and her mother moved to Phoenix, where Faye married Dr. Grant Johnson, a child psychologist. At 16, Michelle, while still in high school, began doing fashion print work and was soon signed by the Wilhelmina agency in New York City. Donen spotted her in a photograph in the fashion biweekly W.

To help start her film career, Johnson moved to L.A. in October. Michelle's mother and stepfather are behind her career "100 percent," she says. In fact, they went over the script for Blame It on Rio and met with Donen before they gave her the go-ahead on the project.

Today Michelle lives in a West L.A. one-bedroom apartment she's been too busy to finish furnishing. "I love the privacy," she says. "But it does get a little lonely sometimes." There is no steady boyfriend in her life, older or otherwise. "I've never had a long-lasting serious relationship with anyone," she says.

Not that she's complaining. A high school friend from Phoenix told her, "You're living everyone's fantasy." Michelle finds that "quite a responsibility." And if that happy burden happens to include the lubricious fantasies of middle-aged males, Michelle is proving she's up to the task.