Liev Schreiber has gotten bigger, and he doesn't like it. This change is not some theoretical emotional state that the hyperanalytical, robustly neurotic actor enjoys teasing apart for a role; it's a physical reality. Schreiber, 41, is carrying 35 pounds of new muscle on his six-foot-three frame, and he's had enough of it. His body creaks like that of a weekend warrior. Formerly attainable yoga positions are now beyond his reach.

"It hurts," he says, shifting in a small seat in a Manhattan diner, where he's sentenced himself to a light lunch. "I'm not kidding!"

Just over a year ago, Schreiber was "smoking a pack and a half and drinking my nuts off" on a Broadway stage eight times a week in a blistering, virtuoso performance in Eric Bogosian's Talk Radio. Then he was cast as Sabretooth, the supervillain in this summer's X-Men Origins: Wolverine, opposite his friend Hugh Jackman. ("The very large and very muscular Hugh Jackman," he says.)

"I started to read blogs in the comic world with things like 'That's the dumpiest, most out-of-shape Sabretooth I've ever seen in my life!'" he says. "They gave me a muscle suit at the beginning. I was so humiliated I thought, I've got to try to do this on my own." Jackman helped him train, putting him on a diet so poultry-intensive that Schreiber jokes they were becoming "the Hitler and Mussolini of the chicken world. But I felt like I owed it to the genre to be big."

Schreiber has been a colossus of New York theater for some time now. As he ambles up Lafayette Street, home of the venerable Public Theater, you feel that this wide stretch of Manhattan real estate is his personal driveway—and not just because he shares a downtown loft with actress Naomi Watts. More than any actor of his generation, Schreiber is a decathlete of the New York stage; he's been the Public's Iago and Hamlet, and the Delacorte Theater's Henry V and Macbeth. When he takes on contemporary material like Glengarry Glen Ross, for which he won a Tony, it's an event.

And now he's having the same impact on Hollywood. The opening of Defiance cuts the ribbon on a year in which filmgoers will get a long look at the ferocious talent that has made him one of theater's favorite sons (he'll play, in rapid succession, a freedom fighter, a crazed mutant, and a drag queen).

It's not as if he hasn't made dozens of movies, ranging from the Scream films to The Manchurian Candidate. What's changed is that Hollywood suddenly seems to get him—the discipline, the commitment, the go-deep enthusiasm for research. When director Ed Zwick saw Schreiber in Talk Radio, he knew he wanted him as Zus Bielski, the tough Jewish resistance fighter in Defiance. The two had an after-show dinner to discuss the role. It's hard to talk to a performer following two adrenalized hours onstage acting like someone who cuts his bourbon with battery acid. As the food arrived, Schreiber unloaded all his misgivings about the project.