“[McCandless] really polarizes people,” Hirsch says. “People are like, Oh, he’s an idiot, or, He’s cool, he’s a hero. It’s up for debate.” But in researching McCandless, Hirsch decided that it’s wrong to make McCandless either a fool or a rebel saint. He became fascinated by “this almost independent, dignified, confident ‘sense of being’ that he had.”

Hirsch has navigated his own career with a similarly confident sense of being. After working steadily doing guest spots on television (on shows as diverse as 3rd Rock from the Sun; NYPD Blue; Sabrina, the Teenage Witch; and ER), he moved into film, quickly racking up credits and recognition with The Dangerous Lives of Altar Boys in 2002 and The Emperor’s Club and The Mudge Boy in 2003. Even if critics weren’t wowed by the films, Hirsch stood out. He made an ill-conceived foray into teen sex comedy in The Girl Next Door (2004), but quickly returned to more serious roles in Imaginary Heroes (2004), Lords of Dogtown (2005) and Alpha Dog (2006).

And then he got the script for Into the Wild. He already knew the basic story well: “I’d seen an episode of 20/20 about Chris [McCandless] when I was about eight years old or nine years old,” Hirsch recalls. “Then I read the book and I was like, Oh yeah, I remember that, I remember that! Then I read the screenplay. I thought I was the luckiest actor ever when I got the part. To be able to make this incredible book—I just loved the book. To go on this adventure, which I knew the shoot would be because we shot in all the real locations. And to work with Sean [Penn]!”

Hirsch’s enthusiasm was tempered by the demands of playing McCandless. One of the biggest challenges of the role is that Hirsch has so many scenes alone. Doing a scene without anyone to react to is usually considered exhausting, but Hirsch views it differently in the case of playing McCandless. “Chris is very comfortable when he’s alone,” Hirsch explains. “He’s very comfortable with himself. The situations where there are people, those are the harder ones for him.”

And so to understand McCandless, Hirsch spent a lot of time solo. “Once you get in the groove of being alone,” Hirsch says, “suddenly you’re very free and you’re very comfortable. It’s a strange thing because a lot of times when you’re by yourself, you’re not paying attention to what you’re doing or how you’re even like. So suddenly when you’re trying to be aware of what it’s like to be by yourself, you’re noticing all these little things. ‘I just said a word out loud,’ or ‘I just did this.’ It’s strange.”

Hirsch also had to transform himself physically. In his last months in the Alaskan wilderness, McCandless withered away. To make his portrayal convincing, Hirsch—who is lean to begin with—went on an intense diet and lost nearly 40 pounds to give him the sunken-cheeked, bony appearance of a man slowly starving to death.

That left Hirsch particularly vulnerable during shooting in Alaska. “It was cold a lot, really cold, especially when I did the weight-loss part of the film,” he says. “It was freezing cold because Sean would be like, ‘Let’s do this scene with your shirt off.’ Then, we’d shoot it for hours. But at that point, you don’t even try to fight it. You’re just like, I’m going to be cold. You just take it.”

Despite how much Hirsch raves about working with Penn and the joys of Wild’s physical rigors, his next project proves he was ready for something a little more fun. He landed the lead role in the Wachowski brothers’ Speed Racer, based on the ’60s cartoon. Into the Wild meant hiking, canoeing and shooting on location; Speed Racer was shot almost entirely on a soundstage using the green-screen process, in which actors have to imagine most of the background and the action. “That was a chance to work in a completely different way,” Hirsch says enthusiastically. “But it was another extreme experience. Extreme tech.”

Not quite ready to accept the fact that Speed Racer, due out May 2008, might launch him into the Hollywood stratosphere, Hirsch sounds more like an average fan effusively talking about a film he’s psyched to see: “It’s the craziest movie. It’s like a really, really goofy, crazy awesome movie.” Not to worry; he’s got seven months to polish his interview patter.

Into the Wild opens Friday 28. See Film.