Listen to film music. A lot of film music!
A lot of film music is among the most interesting and imaginative of the 20th century – they’re just aren’t any scores (listen and transcribe your own!).
Flexibility is or should be the ultimate goal of every musician and composer, and there is no one more musically flexible than a film composer, who has to be ready to compose a jazz theme one moment, a symphonic score the next, a chase scene the next, a haunting shakuhachi theme after that, and make the timing fit to the tenth of a second. Nobody has to have bigger ears and more composing chops than a film composer.
Read books on the history of film music, learn about Bernard Herrmann, David Raksin, Jerry Goldsmith, Miklos Rozsa, John Williams, and many more.
Resolve to learn film scoring at some point: click tracks, timing sheets, condensed scores and orchestrating, and of course all manner of computer and electronic music sequencing and sound manipulation.
Most of all, listen to a lot of film music. Take notes. Transcribe whatever catches your ear as worthy of emulating, copying, imitating, absorbing. Listen for timbres (colors), orchestrations (combinations of sounds), voicings, chords, chord progressions, rhythms, ways to convey visual impressions in music (how do the greats score: tragedy? Joy? A sunset? Imminent danger? High energy? A peaceful, bucolic scene? Love? Playfulness? A majestic landscape? A historical era? A foreign location (Paris. London. Rome. Beijing. Rio. North pole. Moscow. Tahiti. Cuba. Nairobi. Cairo. Scotland. Madrid)?
If you’re between commissions, you could well spend the down time sitting down with a soundtrack and seeing how much you can learn from it.
Build that composer’s compost pile with film music!