Every theme we have ever kept started the same way: not at the desk, but while walking. A good film melody has to survive being hummed — if it cannot live in a voice with no accompaniment, no orchestra will save it.
So the first step is embarrassingly small. We watch the film once without touching an instrument, then go for a walk and hum. Most of what comes out is forgettable, and that is the point — the forgettable ones fall away, and the one that is still there at the corner shop is the theme.
Then the real work starts: the sketch has to survive the picture. We play it against the opening scene, the midpoint, the ending — a theme that only works in one place is a motif for one scene, not a theme for a film. It must bend without breaking: faster, slower, major, minor, fragment, full statement.
Orchestration comes last, and it should feel like casting. Some themes want strings the way some actors want a close-up; others need the distance of a horn across a hall. We try the theme on three or four instruments before committing, and we always record the simplest version first.
You know a theme is finished when you can strip everything away and it still tells the story. Final cue, first hum — if they rhyme, the film has its melody.