NEW YORK -- Her bottom lip. Her bottom lip.

It is, on screen, a plump pink fruit, the focus of her delicate and oddly rounded face, skin the color of milk. And she can inflate it into this amazing pout -- this, this hypnotizing thing. God, she is so beautiful.

But that's only flesh. The source of Jennifer Jason Leigh's real power you can't see and you can't quite define. Talent is too narrow a concept. It's more like a strange kind of seriousness. It drives her acting. It directs her choice of characters.

You see, Leigh likes to take it to the wall. WHAM! Every time. This fragile figure in such shocking circumstances.

Of the 18 feature films she's done, one of her favorite roles was in the 1990 adaptation of "Last Exit to Brooklyn." She played Tralala, the swaggering trollop who gets gang-raped at the end. It is a long, brutal scene -- 20 guys cheering, lining up for their turn -- and when it's over Leigh is stretched out, bruised and naked, on a junked car seat.

"When I shot that scene," she recalls, "everyone was very concerned about me on the set, you know. And I didn't know what the problem was. I was like, 'Hey, I'm fine. Don't worry about me. I'm having a great time.' Because the character is trying so hard not to feel, and she wants it so bad, she wants this gangbang, because she really thinks it is the one thing that will make her feel like she once felt, which was like this queen -- and someone who just didn't feel.

"It's not like a rape in her mind. It is in their minds. In their minds, they're very violent to her, and she thinks they all love her. When I saw the movie, of course, it's the opposite of her experience.

"To see it, and to be that girl up on the screen, was very hard for me," Leigh says. "Because I just felt, I mean, I couldn't, I had a really strong physical reaction. I felt sick to my stomach. I thought I might throw up. I was really shocked and upset by it. Before seeing it, I had no idea how brutal it was."

Polite and businesslike is the way she talks about her work, even this work. She'd rather not be talking about anything at all. She is "very private ... very shy." And these daylong movieland publicity-go-rounds up in some midtown hotel -- this one for "Rush," her latest picture -- aren't her idea of a good time.

"This is hard," she confesses. "Although it's not like I'm having to come up with things to talk about, so it's not as if it was a party. That would be even worse. My idea of hell is going to a party." She smiles. "In my own life, I'm not so comfortable with attention."

Leigh thinks her shyness is "one of the reasons I love acting so much, 'cause when I act, I can completely come outside of myself and become another person and live in that." Which is what she really prides herself on. Dissolving into a character. "Maybe I'm just not so defined as a human being or something, 'cause it's really easy for me to lose myself in other people. And I love that. I want to merge with the character so that I don't know where she stops and I begin, you know? So the more I can do that, the better I feel.

"When I was doing the gangbang in 'Last Exit,' I was very happy, in a sense. It was like the merger was complete. I loved being Tralala. I don't necessarily love watching her. It's a very painful character. But when you play her, you're not feeling any pain." She smiles.

Then comes a hint of something more deeply personal: "When I watch Tralala, I see a lot of myself in her. I mean, I went through a great period in my life where I didn't want to feel. So that's painful, painful to watch for me."

A period where she didn't want to feel? Perhaps that has something to do with the 1982 death of her father, the actor Vic Morrow, in a helicopter accident on a movie set.

But this is something Leigh will not discuss publicly. Bring it up and she slams shut.

"I can't talk about my dad at all. It's just too painful a subject," she says, hushed and flat. "It's a major tragedy in my life and I just can't talk about it. We can talk all about my movies, we can talk all about my life, we just can't talk about my father."

Can she talk about recovering from it?

"I'm still recovering. I mean, I don't think you ever, ever truly recover from something like that. I think you just have to deal with it day by day and do the best that you can do and take what life gives you and try to learn from it and make the best of your life. But I think you're always, you're always dealing with it."

That is her standard response. And that's the end of it.

Although you want to push because you want to know, you want to understand the rich totality of her, to get close because her acting really makes you want to get close, you still have to admire a celebrity -- especially these days -- who insists on keeping private things private.

So this, then, will be a conversation about acting, with one of America's most earnest and most respected young actors. (But not the most honest. She says she'll turn 30 next month, but all the reference books say she's four years older.)

Leigh won a couple of 1990 critics society awards for her work in both "Last Exit" and "Miami Blues," but her name and face probably remain vague to most moviegoers. That's about to change. On Friday, the intense cop drama "Rush" opens in the Washington area. It has already opened to enthusiastic reviews in New York and Los Angeles. Leigh and Jason Patric play a couple of Texas undercover narcs who get addicted themselves. MGM is pushing "Rush" for Academy Award consideration. It might even be a hit.

Which could finally make Jennifer Jason Leigh a big star.

Which isn't a prospect she savors.

"I'm pretty anonymous," she says. And "I really like being anonymous. I like going around unnoticed. I like to blend into the furniture." She means this, though today the business of promoting a movie has her tiny body in a gorgeous baggy gray suit. "I get to do my work, and people like my work, which is wonderful, and I'm left alone."

'Sluts and Nuts' In "Flesh & Blood" (1985), she plays a virgin damsel raped -- vividly, on camera -- by Rutger Hauer as a medieval brigand. She learns to love him. In "The Men's Club" (1986), she plays a bordello's most jaded prostitute, a specialist in kink. Her tongue visits Frank Langella's bellybutton. In "Heart of Midnight" (1989), she plays a child-abuse survivor with mental problems. Again, she is sexually assaulted on screen, and later is menaced while in shackles. In William Mastrosimone's off-Broadway play "Sunshine" (1989), she starred as a peep-show stripper.

With a resume like this, it's no wonder one smart-aleck writer was moved to point out Leigh's predilection for "sluts and nuts."

"For me, I grew up being the good girl in the family. And I think being a good girl involves a lot of suppression," Leigh says. She was raised in Hollywood by her mother, a TV writer, and her stepfather, a TV director. Although her professional name always throws people, Leigh isn't related to Janet Leigh or Jamie Lee Curtis.

"Playing people that are motivated by their gut instinct is fascinating to me because I didn't allow myself to do that growing up. It's a way for me, I think, to expose and understand and explore sides of myself that I wouldn't explore in my own life."

On screen, "I don't like playing it safe," she says. "That doesn't interest me. I really like going as deep and as far as I can go, 'cause it's thrilling." In fact, Leigh told an interviewer in 1990: "I could never play the ingenue, the girl next door or the very successful young doctor. That would be a bore."

So what was Leigh doing in last year's "Backdraft," then? As the love interest of hot young Billy Baldwin, the most demanding acting required of her, it seems, was to simulate sex on top of a fire engine. "There was nothing to do, no," she sighs. She played the conscientious young assistant to a shady politician, a simplistic supporting role in a big-budget crowd-pleaser, exactly the kind of part she never takes.

It's the most popular picture she has ever been in. And she's disappointed with herself.

"For years and years and years, you hear people saying, 'You really should tackle that kind of role. You should do that. You've never done that. You've gotta do that.' You know? So {director} Ron Howard was kind enough -- and I respect Ron Howard, I'd love to work with Ron again -- to offer me this role.

"What I learned about myself is, I can only play roles that I have a burning desire to play. That there's a reason I don't play those women," she says. "I mean, I feel bad. I feel that I didn't do a great job in that film. I tried. I did, you know, my research." Even learned a little Lithuanian. "There just, I just couldn't connect with her. I'm really thankful that I had the opportunity. I don't feel like I lived up to the opportunity."

When is the last time you heard an actor apologize for a performance?

"And if everybody says to me, 'You've gotta do this, you've gotta do that,' I don't have to do that! You know? Because I'm not acting to please the world, or to show how much I can do. I'm not acting to prove anything, really. I want to communicate something that I care about, or that I find interesting, or that I'm challenged by in a way that's deep to me, that means something to me. And if those women who are career women and have it all don't mean anything to me, there's no reason for me to play them."

So Young, So Excited Leigh has been acting as long as she can remember. As a child, she says, "it was a way for me to communicate and to step outside of myself." Performing in school was "where I felt the most alive, the wholest, the happiest." When she got older, she studied with master teachers Lee Strasberg and Stella Adler.

But her first movie role, for which she says she dropped out of high school and never went back, was in an ignominious 1980 slasher film titled "Eyes of a Stranger," starring Lauren Tewes of "The Love Boat." Leigh played Tewes's teenage sister, blind, deaf and mute as a result of a childhood rape. "That was the audition -- walking around the room pretending you're a blind girl," she recalls, laughing. The film's climax had Leigh stumbling around an apartment, fending off a maniacal killer.

Even then, she was a stickler for preparation. She learned sign language and, less necessarily, Braille. She beams when she recalls this.

"It was my first job, and I was sooo excited. I just sat in this chair, I remember, in this strange room, I sat in this chair the entire shoot, like in between takes and stuff. And I just didn't talk to anybody," she laughs. Her character, after all, was blind, deaf and mute. "I mean, it was an absolutely terrible film. But for me, it was this great experience. It really was. I'll always remember it fondly because I was so happy to have my first real job, and to have one that required so much research.

"Also I had this money for the first time in my life -- money that I earned -- so I flew my family in. It was shot in Miami. And I took them out to dinner and bought them presents. I was so excited."

In 1981, Leigh got the lead role in a TV movie, "The Best Little Girl in the World," about anorexia. She lost 18 pounds for it, reducing herself to a sickly 68. Even her doctor got worried.

But she made her first real impact on movie audiences in "Fast Times at Ridgemont High" in 1982, an intelligent teenage comedy now notable especially for its unbelievable collection of then-unknowns such as Sean Penn, Judge Reinhold, Forest Whitaker, Eric Stoltz, Phoebe Cates and Nicolas Cage.

"We were all so young, so excited to be doing a movie," she says. "{Director Amy Heckerling} cast this amazing group of actors who were really, really serious about doing this film. There were no egos. We were all just so happy to be there. Phoebe and I are still very, very close friends."

Leigh played a cute but naive teenager eager to learn about sex. Cates was her older, more worldly chum, happily instructing her on oral sex techniques in the school cafeteria using carrots. Leigh's character lost her virginity on screen. Later, Leigh was displayed fully nude.

Notwithstanding her "good girl" credentials, Leigh has never seemed to mind having her flesh exposed. It happens rather often in her films. She insists that she doesn't think about her own body when she's inside a character. "My ego has nothing to do with my acting except in that: Am I being as truthful as I can be? Am I serving this character as honestly as I can? Am I losing myself in her as completely as I can? Not: How good do my thighs look in this dress? I don't give a {damn}. I really don't. Unless the character gives a {damn}. It doesn't have any meaning for me."

Shop Talk The role of rookie cop Kristen Cates in "Rush" is perfect for Leigh. Plenty of work to do. For one thing, she adopts a convincing Texas twang. Then there's all the research, the time spent with novelist Kim Wozencraft, who based "Rush" on her own experiences as an undercover narcotics officer and drug abuser.

"I also spent a long time talking with drug addicts. People who are currently using, people who hadn't used for years," Leigh says. "What I was looking for was basically how the addiction grew, and the feelings on the drugs, the feelings after the drugs. For example, 'How did you feel three hours later? How did you feel the next morning? Were you cold, were you hot, were you sweaty? Was your mouth dry?' All these things help me in terms of my behavior." This is a movie where authenticity counts. Kristen is forced to shoot up heroin. Kristen gets so strung out she crawls around her apartment looking for cocaine crumbs in the carpet.

Has Leigh ever experimented with drugs herself? "Not really. I'm too much of a control freak." She pushes forward a smile. "I really like to know what I'm feeling and why I'm feeling it. That's important to me. And I'm one of those people that when they get sick, they get really scared." She laughs uneasily. "Think I'm going to die. So yeah."

Surprisingly, the toughest thing Leigh had to do in "Rush" was run. Her character is supposed to have been a track star. "Because I'm not an athlete by nature, I had to learn how to run. And I had this great coach, this ex-Olympian named Lazslo Tabori. And for three months, every morning, I was learning to run. I was home every day afterwards, lying up for half an hour with ice packs. It was a very painful, painful thing for me. I pulled muscles, I pulled tendons, I got shinsplints. I was a wreck.

"So I'm very proud of the running in the movie because I look at it and just, like, can't even believe that that's me."

In developing the relationship between Kristen and her mentor/partner/lover, played with moody intensity by Patric, Leigh found herself having to adapt to his and director Lili Fini Zanuck's different working styles.

"On some movies, like 'Miami Blues' for example" -- in that, she was a naive call girl who runs off with a lying, murdering snake played by Alec Baldwin -- "Alec and I would be rehearsing all the time together. We'd be off in another room doing improvisations and rehearsing and throwing things at one another." She's grinning. "And it was very collaborative. That was a ball.

"And with this, Lili doesn't really like any kind of improvisation. She knows exactly what she wants. And Jason doesn't really like to rehearse. So it was different. It was sort of in-the-moment, finding it and believing it."

If Jennifer Jason Leigh doesn't become a household name in 1992, it will at least have been a great year for her fans. The last film she shot -- and the explanation for her short, dark hair after years of blondness -- is a thriller by Barbet Schroeder, "Single White Female," which isn't due out until August.

"I play a girl who is borderline psychotic," she says. "And she just wants to merge with someone so badly. And she decides she wants to merge with her new roommate, Bridget Fonda. But Bridget's character doesn't really want to merge. So it leads to some problems."

Wow.

"I just loved playing the heavy."