In the mirror, Tobey Maguire almost looks like a regular person. It's 8 a.m., and he's sprawled out in a makeup chair on the closed set of Spider-Man II, wearing gray sweats, shorts and sneakers. He's not exactly larger than life — maybe 5 ft. 8 in. His hair is goofy and tousled. His voice is hoarse and has a touch of Californian dude-ulosity in it. But the chair swivels around, and you notice the eyes, luminous pearly gray lasers that go right through you. Then you remember: Oh, yeah. Right. Movie star.

It was Ernest Hemingway who first described the subspecies of movie star that Maguire belongs to, calling them "the great American boy-men." They're the ageless, fresh-faced whippets of the silver screen — slim of build, dazzling of smile, androgynous of gender. Leonardo DiCaprio used to be one; Elijah Wood is still one. Now Tobey Maguire is breaking ranks. In his new movie, Seabiscuit, he grows up.

Thus far Maguire has made a career out of playing dreamy-eyed kids. He was a desperate adolescent in The Ice Storm, an undergraduate wunderkind in Wonder Boys, an innocent Candide in The Cider House Rules, and lest we forget, a secretive high school super-nerd in last year's blockbuster Spider-Man. But don't confuse him with any of those losers in real life. Offscreen, Maguire is as tough as nails, managing his resume as well as any other star in Hollywood. Look at the arc of his career. It's as perfect as the part in Peter Parker's hair. At 28, he has worked for a string of A-list directors that would make James Lipton weep, including Woody Allen, Ang Lee, Terry Gilliam and Curtis Hanson. How does he do it? Where's his Weekend at Bernie's? "I wait," says Maguire. "I don't want to work as an actor just because I haven't worked in six months. I want to only do things when I really want to do them, and if they only come along every year, year and a half, then that's fine."

Maybe Maguire is a control freak because he never had much control over things as a kid. His parents were 18 and 20 when they had him, and split up two years later. Dad was a cook; Mom was a secretary. Maguire grew up ping-ponging between them, moving from state to state. Often he was so nervous he threw up in the morning before school. Then everything changed. He was in junior high, and his mom wanted him to take a drama class. She bribed him with $100. After that, it was commercials — his first onscreen performance was in an Atari ad — and TV shows like Blossom and Walker, Texas Ranger, then movies. His last year of formal schooling was ninth grade.

Maguire exercises the same kind of control over his personal life. He doesn't drink — in fact, he has been in Alcoholics Anonymous since he was 19. He doesn't eat meat. He's all about boundaries. Last year Autograph Collectors magazine ranked him No. 4 on its list of the world's worst autograph signers. Maguire has exactly one vice, illegal Cuban Cohiba Robusto cigars. On this morning, he smokes two before noon. In a three-hour interview, he makes exactly one joke. Asked if he is worried about the cigars messing up his million-dollar voice, he pretends to be offended. "A million?" he says, as if that's not enough. (Fact is, he's reportedly getting $12 million for Seabiscuit and $17 million for Spider-Man II.)

One side effect of his closed-border policy is the rumors that circulate about him. A quick rundown: Is he still dating Jennifer Meyer, daughter of Universal Studios head honcho Ron? Probably — but he doesn't talk about it. Does he really have a bad back that limits his ability to do stunts? Yes, but it's feeling fine these days, thanks. Did Columbia Pictures try to replace him in Spider-Man II with rival boy-man Jake Gyllenhaal? And did he have to beg his way back into the Spidey suit, with the help of the aforementioned well-connected girlfriend? No comment beyond "And here we are." Plus a zap with the lasers to say, Move on.

All that discipline pays off onscreen. Maguire is one of the world-champion under-actors. He never overdoes a scene, never overplays a reaction, never bullies you into feeling an emotion he doesn't earn. "One of the things that distinguishes Tobey as an actor is his ability to do more while appearing to do less," says Hanson, who directed him in Wonder Boys. Look at him once, and it's hard to tell he's even acting. His face barely seems to have a muscle in it. But you can't look at him once. There's something alert and alive in those eyes. There's something mysterious and curly going on around the corners of his mouth. It's sly, but not ironic-sly, like he's messing with you. It's more like you and he are messing with everybody else, together.

Seabiscuit is the acid test for Maguire. For one thing, he has to find out if people can look at him and not see Spidey. As Christopher Reeve found out, once you put on the tights, it can be hard to get them off. For another thing, Peter Parker was a high school kid. This is the first time we'll see Maguire as a man playing a man. "There probably won't be a lot more things I do where the character is, like, a virginal, innocent, sexually naive kind of guy," he acknowledges. "Only because being 28, it's just getting laughable to me." Not that that's why he took the part, he adds. "But am I glad I didn't have to do four or five scenes where I talk about being a virgin or have trouble talking to girls? Sure."

Based on the best-selling book by Laura Hillenbrand, Seabiscuit tells the true story of an ornery, undersize, beaten-up Thoroughbred who becomes a champion in the 1930s. Seabiscuit is discovered by a broken-down cowboy (Chris Cooper) and a rich dilettante named Charles Howard (Jeff Bridges), who nurse him back to health. They need a rider who can handle him. Enter Maguire as Red Pollard, a bitter, washed-up jockey who was abandoned by his parents as a kid, then grew up too tall to make the big time. Along the way, Pollard lost the sight in one eye, and by the time Team Seabiscuit finds him he's boxing on the side (and losing) to make ends meet. Writer-director Gary Ross, who directed Maguire in Pleasantville, wrote the role for him, but Maguire had to be physically transformed for it. His hair was dyed red and piled up on his head in an angry Brylcreemed ziggurat, and he had to slim down to 140 lbs. and 6% body fat.

Howard recognizes Pollard as a kindred spirit for his stubborn steed and takes him in, becoming a kind of surrogate father to the angry, abandoned Red. "You kind of see his safe, secure, loving world get crushed, and you see his boyishness just drain out of him," Maguire explains. "He toughens up and withdraws and keeps himself guarded. And then you see that melt away again as he becomes part of this family." Watching Red slowly learn to trust those around him, you realize that Maguire has taken his standard boy-becomes-man routine and subverted it. He plays a man who, having been robbed of his childhood, becomes a boy for the first time.

There's enough of Pollard in Maguire — the guardedness, the tough childhood — to make a journalist's Spidey-sense tingle, but Maguire is emphatically not into making those kinds of connections. "My trust issues? I may have some," he says. "Having my guard up? I may do that. I don't sit there and pick out experiences or traits of mine. That would be more for Gary Ross to do. For me, I'm just playing the part."

Maybe so. But even Maguire can't keep his guard up forever. In the best scene in Spider-Man, Parker realizes he has superpowers and is so psyched about it that he goes running off over the Manhattan rooftops. In a way, Maguire plays the same scene again in Seabiscuit. Howard takes his new horse and his new jockey out to the country to open them up and find out what they can do, and we see for the first time what a miracle they have on their hands. Talking about the scene, Maguire sounds almost mesmerized. "I think what plays there is somebody who's just kind of desperate and hungry and broken down, and Seabiscuit and my character together reawaken themselves. He feels like when he was a kid and he rode horses and he loved it, and it felt right to him, like what he was supposed to do. And Seabiscuit's galloping, and I think it's just, it's just like that kind of epiphanic release in one's life, where you're just like ahhhhhh!" It's one of the rare moments when Maguire isn't holding back, when you can tell he knows one of those secrets every actor has to learn — that sometimes the essence of control is knowing when to let yourself lose it.

• Lev Grossman/Los Angeles