In September, composer Michael Giacchino received an exciting, intimidating job offer: Disney wanted him to compose the score for "Rogue One: A Star Wars Story," a dream for the lifelong fan of the franchise, who even played a Stormtrooper in "The Force Awakens." However, there was a catch. He'd need to complete the work in just four and a half weeks.

"I mean, we were literally planning a vacation when I got the call," Giacchino, whose won an Academy Award for the score of "Up," told Entertainment Weekly. "At the time, it left me with literally four and a half weeks to write. So it was one of those decisions where you’re like, okay, well … And I was talking to my brother about it. He goes, 'Oh, come on. You’ve been writing this score since you were 10! You can do this.'"

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Alexandre Desplat, known for his work on “The Twilight Saga: New Moon" and “Harry Potter and the Deathly Hallows,” was originally hired to write the music for "Rogue One," but extensive reshoots pushed back the post-production schedule, leading him to drop out.

"It’s not really [enough time]," Giacchino explained of the narrow window for scoring the film. "But you work with the time you have,. I’m pretty good at focusing and getting down to business. I saw the film and I really, really, really enjoyed it, so there was no lack of ideas or inspiration, that’s for sure. The only worry the whole time for me was just the schedule."

"Aside from all that it was really fun to do," he continued. "It was really fun to come in every morning and just look up at the screen and see Stormtroopers running around … Part of me was stressing out about the timeline. But the other part of me was just like, 'This is the greatest thing ever! This is so much fun!' So I really tried to just keep all the negativity, whatever, or be away from it, just so I could look at it and enjoy it. I wanted to make something that I would be happy with."

Giacchino has some big shoes to fill, as "Rogue One" his the first "Star Wars" film without a score by legendary composer John Williams. Giacchino himself has scored a number of other Disney properties, most recently "Doctor Strange." In fact, he said that the call from Lucasfilm about "Rogue One" came "the day after my last day of scoring with 'Doctor Strange.'" He also scored the rebooted "Star Trek" films, Pixar's "The Incredibles," "Inside Out" and "Ratatouille," and provided music for "John Carter" and "Zootopia."

Directed by Gareth Edwards, “Rogue One: A Star Wars Story” opens Dec. 15 in the United Kingdom and Dec. 16 in the United States.