British beauty Patsy Kensit -- In ''Twenty-One,'' the actress finally graduates to a leading role

We meet her in a facial mask, chat with her on the toilet, and watch her ricochet from a married man to an impotent junkie to an ebullient reggae singer. She admits she’s a ”minor-league s—,” yet when Patsy Kensit, as effervescently honest Katie in Twenty-One, tells us she’s ”not looking for the greatest intimacy…I’m looking for a straightforward f—,” she manages to sound more like Grace Kelly than like a female Andrew Dice Clay. Maybe it’s that convent-school background seeping through.

”I do think convent-school girls come away with something,” muses Kensit, 23, popping olives and sipping Coke in the Four Seasons tearoom during a Los Angeles sojourn. ”Maybe it’s a voracious sexual appetite.”

Let’s hope the nuns at Kensit’s alma mater, St. Catherine’s in Twickenham, England, didn’t hear that. ”And God help me if they see Twenty-One,” says Kensit, looking squeaky-clean in a white T-shirt, jeans, and a straw hat. ”They thought my finest hour was playing the fairy at the end-of-term play.”

Twenty-One, the sensation of the Sundance Film Festival in February and recently the toast of tout Paris, is no fairy tale. In the British feature, opening gradually across the U.S., Kensit’s 21-year-old Katie confides directly to the camera in an intimate tour of her hectic existence: dead-end jobs, dead-end boyfriends, and an unraveling family life. The movie has received mixed notices, but Kensit has won raves.

”Katie may be the opposite of how people want to see women in films,” Kensit says. ”She’s not ‘the girlfriend.’ But I’d rather see a movie like this than one that makes a Cinderella story out of prostitution. That is a male fantasy.” Is Kensit anything like her role? ”No,” she says. ”I’m far too timid to conduct myself the way that Katie does.”

Kensit made her film debut in 1974 at age 4 when a family friend helped her get cast as Mia Farrow and Bruce Dern’s daughter in The Great Gatsby, but she has never been taken seriously as an actress. Twenty-One director Don Boyd calls her first big-screen splash, 1986’s Absolute Beginners, ”a spectacular failure. My first thought (when casting Twenty-One) was, ‘Oh no. She won’t do at all.’ But on our first meeting Patsy intelligently explained the trouble she had with her image. She was down-to-earth, and I liked her immediately.

”Patsy wasn’t really known as an actress so much as ‘a face,”’ says Boyd. ”The tabloids treated her like a bimbo — which in Patsy’s case was quite unfair.” One reason was Kensit’s former job as lead singer in her brother Jamie’s band, Eighth Wonder, which had a No. 1 single in Europe, ”I’m Not Scared,” in 1988. ”People think that if you’re involved in music, you can’t have any brains,” she says.

Since then she has had minor roles — ”the girlfriend running behind Mel (Gibson) screaming” in Lethal Weapon 2; Kiefer Sutherland’s fiancée in Chicago Joe and the Showgirlbut Twenty-One is her proving ground. Shooting it was ”like having six weeks of therapy,” she says, because she was trying to get over a split from her husband of four years, Dan Donovan, former Big Audio Dynamite keyboardist. She has since become involved with Simple Minds’ lead singer, Jim Kerr, ex-husband of the Pretenders’ Chrissie Hynde, and has completed more therapeutic filming: Time Bomb, a thriller with Michael Biehn; Beltenbros, set in Franco’s Spain; and Blame It on the Bellboy, a Disney comedy with Dudley Moore.

”You get so spoiled in America,” Kensit says. ”Actors are treated like royalty: ‘Can I get you this? Can I get you that? You can kick me if you’d like.”’ So it was a relief recently to get back to London for a BBC production of George Eliot’s Adam Bede. ”It’s a wonderful leveler to work in England,” she says. ”The makeup girl tells me to get her a cup of tea.”