Sean Penn has wanted to make a movie based on Jon Krakauer's 1996 book "Into the Wild" since the moment he finished reading it. The true story of Christopher McCandless, a recent college graduate who in 1990 cut ties with his family and embarked on a two-year odyssey that ended tragically in the Alaskan wilderness, struck a major chord with the actor/director. And while it took him years to convince McCandless' parents and sister to give their blessing to the project, it took only a matter of hours for him to secure longtime friend/Pearl Jam frontman Eddie Vedder to write new original material for the movie's soundtrack. § On it, Vedder plays nearly all the instruments and explores more of an acoustic, stripped-down musical approach than normally heard on Pearl Jam albums. The soundtrack debuted in September at No. 11 on The Billboard 200 and has sold 95,000 copies in the United States, according to Nielsen SoundScan. First single "Hard Sun" is No. 28 this week on the Modern Rock chart. § With "Into the Wild" garnering strong reviews and whispers of Academy Award nominations, Penn and Vedder talked with Billboard about their creative partnership. The pair will expand on the subject during a keynote interview Nov. 1 in Los Angeles as part of the Hollywood Reporter/Billboard Film & TV Conference.

If you can recall, at what point did you start thinking about what kind of music would be in the movie?

Sean Penn: I'm going to guess that it was right from go. But in terms of really identifying that I was going to structure transitions to be told in song, that was when I first started to ask myself, "OK, what are all the components of things I've been thinking of for the last 10 years?"

Did you have actual songs in mind for those transitions?

Penn: Oh, yeah. I had model tracks throughout. There was Neil Young's "Hey Hey, My My," Cat Stevens' "Miles From Nowhere," Joe Henry's "King's Highway" and Philip Glass' "Cloudscape." That was less in a transitional state than it was in a visual one. There was Lynyrd Skynyrd's "Simple Man" too.

When you pulled the trigger on asking for Vedder's involvement, did you show him a script?

Penn: I don't even remember whether I gave him a script at all. By the time I went to him, I had a rough cut of the movie. He was in Hawaii when I tracked him down. He got a copy of the book and read it. He called up very invested already. He really connected with it. I said, "Call me when you get back and I'll come up to Seattle," and that's what happened. I brought up like a three-hour-and-15-minute cut of the movie, and we sat and watched that. His words were, "It's on," and that was it.

Eddie Vedder: The film ended and we shared a moment of silence, because it was heavy. I think I just asked him, as I'm reaching over to light a cigarette, "What do you want?" And he said, "Whatever you feel. It could be a song, it could be two, it could be the whole thing." So I went in for three days, starting the next day, and gave him a palette of stuff to work with. And then he started choosing. Immediately he had a few things he put in. I wasn't expecting that. After that, then it was really on. What I gathered was, the songs could now become another tool in the storytelling, especially when you have shots of the young man solitary. In a way, it's offering a window into what he's going through intellectually and emotionally without having to have him talk to himself [laughs].

Click here to read the full interview, including Vedder's writing process for the project, contributions to the film's score, people's reaction to the hard-hitting film and more.