Sean Penn spent a decade preparing the film adaptation of Jon Krakauer's 1996 book "Into the Wild." But it only took him a few hours to secure the services of Pearl Jam frontman Eddie Vedder to write the soundtrack for the project.

The pair's creative partnership was the subject of a keynote address today (Nov. 1) at the Hollywood Reporter/Billboard Film & TV Music Conference in Los Angeles. Vedder and Penn were joined in conversation by Michael Brook, who wrote the "Into the Wild" score.

"The script had been structured to have songs carry some of the narrative," Penn said. "We'd use a lot of Michael's stuff to temp to during the shoot. I just asked [Vedder] initially for a song [and] rather quickly he started sending things down. The most important moment in the collaboration was just sitting at his place watching the movie together. He'd send these songs down and they'd just work perfectly."

Brook likened the process of blending his music with Vedder's songs to "a kind of conversation" and said he was delighted "how much experimenting there was, which is what I always like best. Some of the music I wrote just reading the book. A lot of that we threw away, but some of it actually wound up in the film. At the beginning, you're not looking at the picture so much. You're just thinking, 'What is the atmosphere?'"

For Vedder, who previously turned down persistent requests from Penn to act in one of his films, writing for "Into the Wild" was akin to finding "the perfect wave. Sean had found the perfect wave by the time he talked to me about it. It was just a pleasure to surf this perfect wave. All I had to do was get back on the boat and sing for my supper."

He added that working with Penn and the family of the story's main character, Christopher McCandless, top of mind was a welcome creative challenge. "In our group, or the way we're used to doing it, we have five guys and we're all kind of the boss," Vedder said. "This felt more like, these people I felt responsible to were the boss. I felt really comfortable in that position."

Vedder and Penn have given only a handful of interviews about the project, but joked about their true reason for appearing at the conference. "Our motivation in being here today is primarily those briefcases full of cash from Billboard," Penn said with a laugh.

For Billboard's in-depth interview with Sean Penn and Eddie Vedder, click here.