Talk about a fun assignment: Last night, the Los Angeles Philharmonic played Alan Silvestri’s music for the 1985 film Back to the Future “live to picture,” with the estimable David Newman conducting. It was a near-sellout with more than 16,000 people attending, and the crowd cheered every iconic moment in the time-travel romp starring Michael J. Fox. I interviewed the composer about his memories of scoring Back to the Future — which was only his second opportunity to write for orchestra — and why he wrote another 20 minutes of music specifically for these live presentations.