Did you read the book before you started the film? Emile Hirsch: Oh, yeah. When I talked to Sean, he’s the one who got me to read the book right away, and I read it that night. Then over the next four months Sean and I would just go out drinking, go hang with the family, go out to eat, and get to know each other.

For a young actor, hanging out with Sean Penn must have been something. It was amazing. He has just a very, very funny, hilarious personality. He’s got a tremendous serious side, too, and is a man of conviction and integrity, but he also has a very light spirit, which is a lot of fun.

America should have a starring credit in this movie—it plays such an important part. It’s a character in the film. We saw so many different places. We saw this unforgettable slice of the American heart. We were taking helicopters and private planes over the Grand Canyon, taking Jeeps down into the canyon to get to the Colorado River, going on barges for two weeks, camping in the canyon, kayaking on the Colorado River, doing rapids.

That was really you on those rapids? Yeah.

They were rough. Were you scared out of your mind? It was pretty crazy. It was an exhilarating moment. I would be curious to see what my pulse was.

And, of course, the grizzly bear scene. Between the grizzly in this movie and the monkey in your next film, “Speed Racer,” which was more challenging to work with? They pose a different set of challenges, each one of them. The grizzly bear is imminent death, and the monkey is just being maimed for life. You can live through a monkey attack. You might be missing an eye or a testicle, but a grizzly bear, you’re probably not going to live.

Chris was a complex person, and aside from the idea of this solitary man on a quest to find himself, he had some anger driving him away from his family and the life he knew, don’t you think? I talked to his sister, and he was a really angry guy who really resented a lot of stuff that happened to him in his childhood and as a young man. So, you know, he was trying to be noble, but he was conflicted by this. It was his Achilles’ heel. He was such a great guy in all these ways, but he just couldn’t let this go; he could not let it go with his parents.

Do you think that wanderlust is as alive now as it used to be—not to McCandless’s extreme, but still there? I know a lot of people still do that kind of stuff. If you go to the bus [where McCandless stayed] now—the real one—there are huge journal entries of other travelers who have gone to the bus.

I just read that someone took his real boots and left theirs. Since you have such a personal connection to this, how does that strike you? It disappoints me, but I don’t think that if McCandless knew someone took a pair of his boots he’d stress about it. He’d probably be like, “Why are you putting this value on material things like this?” I don’t think he’d be like, “Leave the boots for my sanctity.” That wasn’t the kind of guy Chris was.

It was important to Sean Penn that whoever played McCandless be on the “cusp between being a boy and being a man.” Did you feel that kind of transformation personally? That’s so presumptuous of me to say, “I’m a man now.” I just think that’s something a not-man would say. [Laughs.] I feel like it was a challenging experience, and I feel like I learned a lot from it. But as far as me announcing the fact that I feel like a grown-up … I mean, that’s just bogus.

But did you sense the metamorphosis in some way within yourself? Maybe, I don’t know. I feel a little tougher, a little stronger on the hide. All I know is that we all worked our butts off making [the film]. We made it not being lazy.

How does it feel being “the next Leonardo DiCaprio,” as the phrase now goes? It’s great. People are moved to say those things, and I’m not going to complain.

But I bet if I try to get you on the phone this time next year, it might be a little harder. That’s funny. A lot of you guys have been saying that.