Originally, Penn wanted to cast unknown actors in the film. But with so little time to prepare, he chose veterans such as Vince Vaughn and William Hurt—people "who weren't gonna need a lot of nurturing." The toughest decision was casting the part of Christopher McCandless.

PENN: I'd seen Lords of Dogtown and thought Emile Hirsch's performance was intriguing. So I spent four months of on-again, off-again meetings with him to try to get a sense of whether he was gonna be ready to make the kind of commitment that was necessary for this. This was gonna be a brutal kind of thing. I had no idea how he would blow our minds.

EMILE HIRSCH, 22, actor: Sean said, "I've got this book and I want you to read it." I'd seen a segment [about McCandless] on 20/20 when I was nine, and it made a big impression on me. The idea that a person would go off and be alone and have to suffer and have joy alone is something that to a kid is almost impossible to understand. And that was the reason I never forgot it. So I read the book the next day and flipped for it. There's this wanderlust, this unexplored side; everyone wants to go out on the road and have an adventure, you know? I was 21. My life had been kind of flat and uneventful at the time. The book just kind of reawakened what was possible. It was like On the Road without all the Benzedrine.

WESTERBERG: I got to know Alex pretty good. [Emile] was a lot like him, same size and build and came from growing up with money and found some adventure in this. He tried to get totally involved in it.

WALT: You see Emile up close and he doesn't look a lot like Chris or have the same voice sound. That's an almost impossible thing. But you see him in the action parts of the film, when he's walking into the wild after they let him off at the trailhead, and, boy, he looks exactly like him.