While Cameron Crowe directed “We Bought A Zoo,” he and his team juggled completing this fall’s “Pearl Jam Twenty” documentary and “The Union,” his film about the making of Elton John and Leon Russell’s 2010 album of the same name, which comes out in January.

“They all fueled each other,” Crowe says, noting that he’s hardly the first moviemaker to multi-task. “To me, the best documentary certainly in the last 25 years was [2005’s] ‘No Direction Home,’ and I heard [Martin] Scorsese did that while he was doing ‘The Departed’.”

For “We Bought A Zoo,” which had a sneak peak this past weekend (read Drew McWeeny’s glowing review here) and will open nationally on Dec. 23, Crowe knew that for the first time in one of his movies, he wanted to focus more on a score than licensed music (although “Zoo” features a number of songs). “I just had the feeling that this was the movie where we could do more score than ever before,” Crowe told me when I interviewed him recently for a piece for Variety (this article includes  a few comments from that piece, but primarily is previously unpublished material).

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Though Crowe considered other potential composers, Sigur Ros’s Jonsi had his musical fingerprints all over the film from the start. The movie’s code name was “Heima,” taken after the 2007 Sigur Ros documentary. Plus, Crowe played Sigur Ros’s music and tunes from Jonsi’s 2010 solo album, “Go,” throughout filming to help create a specific mood. In fact, even before Crowe had settled on Jonsi, he filmed a pivotal scene with Matt Damon while “Sinking Friendships” from “Go” played in the background. Damon plays a widower, who moves his young children to a new home with a decrepit animal park.

“He was part of the DNA of the movie the whole time,” Crowe says. “We were playing him on the set starting with rehearsal...Jonsi kind of claimed the movie long before he knew we existed. Jonsi always captured the adventure of it and the adventure of being alive, the highs and the lows.”

Crowe thought about asking pals Neil Young or Eddie Vedder to score the film (both of whom were on a mix CD Crowe made for Damon), but ultimately felt Jonsi was the right voice. At the same time, Jonsi just happened to catch “Almost Famous” on television in his native Iceland and dropped Crowe an email. The two had previously worked together when Crowe had licensed three Sigur Ros songs for “Vanilla Sky.”

A few days later, Jonsi was on his way to L.A. The first piece composed serves as the movie’s theme. It’s a magical, child-like composition, written on his laptop and played largely on a glockenspiel, that creates a sense of awe and wonder.  Jonsi also wrote two new songs for the film, including “Gathering Stories,” for which Crowe helped with the lyrics.

“I flew to [L.A.] for one week. I wrote all the main themes in a week on my laptop. That week was really important,” Jonsi recalls. He worked primarily in a house on the Fox lot that had 10 offices. “Cameron had one office, I had one office, the music editor had one office...so all the people who were working on the film were only in this house. You didn’t really bump into anybody else. Only these people in this house. It was a really concentrated energy. I fed off that. I got feedback right away.”

That initial feedback was vital since Jonsi had never scored a theatrical film before. “The question was could he write stuff based on this movie and based on these characters as opposed to the huge canvas that he’d always written on in the past,” Crowe told me for the Variety. But Crowe says it became apparent very quickly that he was up to the task.

Crowe even asked Jonsi, given the theatrical nature of much of Sigur Ros’s music if he thought “cinematically” when he wrote. “He just said, no,” Crowe says. “That he shuts his eyes and gets lost in the musical world.”

For Jonsi, the hardest part was writing to someone else’s vision. “You can be egocentric when writing your own music,” he told me for Variety. But when writing for film, “You have to think in terms of how the movie flows, this scene, etc. I learned a lot from it.”

Crowe looks back on the experience of making the movie fondly, but admits he has on regret: “I never got around to the big screening of ‘Pearl Jam Twenty’ for the [‘Zoo’] cast and crew. That would have been the studly move; to gather them all together one night and show it,” he says with a laugh. “But we were all too busy.”

The score for “We Bought A Zoo” comes out Dec. 6 on Columbia.