How the score is the driving element behind ‘Babylon,’ ‘a musical without being a musical’

“Babylon” composer Justin Hurwitz used instruments from the Roaring ‘20s era but infused them with a spirit that was more modern rock ‘n’ roll or even rave culture. They’ve known each other since playing in a band together at Harvard, and Chazelle has always given Hurwitz an outsized role beginning with their early musical, “Guy and Madeline on a Park Bench,” and through the music-centric films “Whiplash” and “La La Land.” So Hurwitz was surprised to learn only recently — at a Q&A — that the director had been developing his idea for “Babylon” for the last 15 years. Likewise, “Babylon” blurs the line between what is traditionally considered diegetic or “source” music, and what is score: characters will be dancing — or snorting mountains of cocaine — to a tune being played by a live jazz band one minute, then accompanied by a variation of that same tune in Hurwitz’s off-screen score the next. When they sat down to plot “Babylon” in the fall of 2019, Damien Chazelle’s script was already filled with musical directions for his longtime composer Justin Hurwitz.