WATCHING Philip Seymour Hoffman (big head, big body, big deep voice) embody Truman Capote (small head, small body, bizarre baby voice) in "Capote," you want to throw every acting award there is at him and maybe a couple of Olympic medals, too. What concentration! What shape shifting!

And the performance transcends mimicry. With ruthless precision, Mr. Hoffman exposes the underbelly of his character's empathy, as Capote exploits his relationship with the convicted murderer Perry Smith (Clifton Collins Jr.) for the sake of his book "In Cold Blood." You are left with a lacerating portrait of the artist as vampire, driven to feed even as he is swamped by self-disgust.

For an actor who holds nothing back on screen, Mr. Hoffman, 38, is strikingly - maybe disappointingly - unhistrionic over lunch near his apartment in Lower Manhattan, where he lives with his girlfriend and their young son. No booming voice, no demonstrations of virtuosity: you would hardly know he was an actor.

You hear two things from people who have worked with him: a) that he is one of the nicest, most humble performers in the business; and b) that on sets he can be unapproachable.

"I wouldn't disagree with that at all," Mr. Hoffman said, evenly, with the faintest wince at the prospect of being portrayed as high-strung. "There are certain jobs, in certain environments, when I'm not as scared. So, therefore, I am who I am, which I think is a pretty decent person.

"But if I'm struggling, if I feel like I'm falling short, I'm incredibly hard on myself. And when you're in front of a camera and in front of people, that's a very vulnerable place to be. I can become difficult. But I'm not demanding. I'm not the guy who's not coming out of his trailer. I'm definitely the guy that will start to swear very loudly at myself."

Those are the moments when you don't want to stroll up and make idle chitchat. Or even eye contact. "I'll think: 'Is there any way I can get private? Is there any way that you all cannot see me?' " he said.

It's strange to hear an actor bemoan the fact that people are looking at him, but there is something deeply private - and all-consuming - about the parts Mr. Hoffman chooses to play. He first wormed his way into the minds of critics and filmgoers in Todd Solondz's 1998 pedophile tragicomedy, "Happiness." Mr. Hoffman wasn't the pedophile. He was the fatso voyeur mouth-breather who would phone his hotcha neighbor (Lara Flynn Boyle) and abuse himself while telling her all the bad things he wanted to do to her. Sitting on the edge of his bed, naked but for his briefs, his huge, pasty gut resting on huge, pasty thighs and with a voice that seemed to gush straight from his bowels, Mr. Hoffman secured a place for himself in the Movie Pervert Hall of Fame.

Not all his performances make you squirm to that degree, but since "Happiness" Mr. Hoffman has tackled some formidable oddballs: a saucy transvestite nightclub singer in "Flawless" (1999); the grief-addled husband of a suicide in "Love Liza" (2002); a drab but monomaniacal gambler in "Owning Mahowny" (2003); an emotionally ravaged defrocked preacher in "Cold Mountain" (2003); and a loutish (and riotously funny) ex-child actor in "Along Came Polly" (2004).

His alcoholic Jamie Tyrone in "Long Day's Journey Into Night" on Broadway in 2003, opposite Vanessa Redgrave and Brian Dennehy, seemed relatively laid-back - until his big scene, Jamie's confession of malice toward his younger brother (played by Robert Sean Leonard), which began groggily and then built to an excruciating intensity. After six months as one of Eugene O'Neill's most prodigiously tormented souls, doing Truman Capote must have seemed like a cakewalk.

Given his endomorphic body type (he complains about his tendency to gain weight too easily) and the way he is cast as people who are not in primo shape, it is surprising to learn that Mr. Hoffman was something of a jock in high school, in Fairport, N.Y., near Rochester. He played baseball, then switched to wrestling. He stumbled into acting only after an injury knocked him off the mat. He's still a sports nut.

"Sporting events are very much like theater," Mr. Hoffman said. "They do the same play, but it's different every night. There's both discipline and creativity. There's visualization: In wrestling, you're on top and you think, 'Okay, if I hit his arm there I can get around here,' and you see yourself do it. Same as acting. They have a goal and they go after it, and out of going after it their character will be revealed."

Their character: what they are made of. Whether they go for broke, put everything on the line. You can see why the stakes for Mr. Hoffman are so high.

Mr. Hoffman's first big high school role was ... Get ready. Willy Loman in "Death of a Salesman." He said he knew it was way beyond him. But what, he asks, is the sense of doing something you know you can do? "It's a law of nature," he said. "The higher the risk, the more you'll be compelled to succeed. So you look for something that takes you to a scary place, something unknown, that will take up all your time, all your head - your everything."

Local recollections (there have been plenty since Mr. Hoffman's star has risen) suggest that his doomed salesman stunned audiences with its risk-taking - although he now admits that he cribbed a bit from another go-for-broke actor, Dustin Hoffman, whose Loman he saw on Broadway when he was down auditioning for New York University. Recently, he was thrilled to hear from Mr. Hoffman (Dustin), who called to laud his Capote. "What I didn't tell him," Mr. Hoffman (Philip Seymour) said, "was that his performance excited me so much that I left the theater and did that cheesy movie thing where I found myself running because I was so excited and happy."

In concert with his N.Y.U. theater curriculum, Mr. Hoffman studied at Circle in the Square, where he credits two teachers for two different approaches: one action-oriented ("high stakes, life and death stuff," he said), the other more internal and classically Method. "It seemed to me that one without the other meant bad acting," he said. "But the two together meant you had a shot at doing well."

For Mr. Hoffman, a great deal of thought goes into reconciling those two approaches. "A lot of acting teachers will say, 'Don't think, just do,' " he said. "But I had a teacher say to me: 'No, no, no. Think. Make sure you're thinking all the time. Be a thinking actor. Please be thinking. Don't stop thinking.' "

What about coaches who tell their players they're thinking too much?

"Yeah, but those players are thinking about how they're going to fail," Mr. Hoffman replied. "Thinking doesn't mean being self-conscious. It means watching the ball and thinking about how it's going to curve. Then you might actually hit it."

Mr. Hoffman's first big hit came in "Scent of a Woman" (1992), in which he played Chris O'Donnell's obnoxious prep-school pal. ("Philip S. Hoffman" appears last among the actors' names in the opening credits.) Now he looks back on his early screen performances and finds them unfocused, overly busy. But in 1996, he had a vivid scene as a blowhard gambler in Paul Thomas Anderson's first feature, "Hard Eight," the beginning of a close friendship with Mr. Anderson, as well as roles in all Mr. Anderson's subsequent films.

His performance as a sweet but rather grotesque hanger-on who is madly in love with Mark Wahlberg's porn star in Mr. Anderson's "Boogie Nights" (1997) was a breakthrough in his process - in learning to enter completely into the head of a character, on camera and off. "Ultimately, you have to create such a layered understanding of how your characters function and why they function that you have the confidence to step forward and open your mouth," he said.

It's important to say that total immersion can have its downside. Mr. Hoffman can plunge so deeply into his roles that his acting can seem unmodulated, his characters off-putting. Flailing around in "Love Liza" (written by his brother, Gordy), his character's suffering is so extreme that he becomes hard to watch. His Mahowny in "Owning Mahowny" is a man stripped down to pure addiction, with barely a flicker of awareness. You could argue that it's a mark of Mr. Hoffman's integrity that he does nothing to leaven his characters' obsessions. But this can also result in monotony - in a lack of drama.

And here is something you don't hear every day from movie stars: In the editing room for "Capote" (which he co-produced), he fought with the director, Bennett Miller, to make his character less attractive. "You could easily cut that film and not show him in as harsh a light," Mr. Hoffman said. "And I said: 'The way toward empathy is actually to be as hard as possible on this character. The harder you are, the more empathy you'll gain, ultimately, by the end.' "

I tell him I don't entirely follow this.

"Meaning, you might still have a very strong opinion about him, but you will definitely understand why he would be in the emotional distress he was in, if you really, actually see him act in ways that are reprehensible," he explained.

"I think deep down inside, people understand how flawed they are. I think the more benign you make somebody, the less truthful it is."

This is certainly the way an actor who beats himself up so much would think - that the way to an audience's heart is through cruel self-exposure. It's this sacrifice, Mr. Hoffman maintained, that separates the great actors from the journeymen. Training for "Capote," he said, was like going to the gym for hours every day, pushing his body and his voice to places he wasn't sure were even within reach.

He's trying not to think too much about the Academy Awards race, which could come down to a contest between two very different kinds of gay characters: Mr. Hoffman's preening, loquacious dandy versus Heath Ledger's Marlboro Man of few words. In the meantime, he's taking a break after completing a far more athletic sort of part - the heavy in "Mission: Impossible 3," opposite his "Magnolia" co-star, Tom Cruise. He loved the physical training and hanging out with stunt people, but this was a high-paying exception to his usual regime, and to the pressure-cooker approach that makes him alternately awe-inspiring and a pain.

"It's not pleasant living in those feelings, living in those thoughts," Mr. Hoffman said. "It can make for difficult days in which you're difficult to work with. And it can make for very, very beautiful days. The intimacy you achieve with your director or other people on a film when you're going through that - it can be some of the great stuff of your life."

Some of the great stuff of your art, too - if you can manage to put up with people looking at you.