A soundstage at Cinecittà, the summer of 1968. Sergio Leone has decided that for this picture, the music will be composed first. Ennio Morricone is at a piano weeks before any of the actors arrive in Spain. The themes are not cues yet — they are characters. Cheyenne gets a banjo, lopsided and grinning. Harmonica gets a harmonica, of course — Charles Bronson's whole inner life compressed into one bent note. Frank — Henry Fonda playing against thirty years of decency — gets a baritone guitar and Edda Dell'Orso's wordless soprano hung above it like weather. Leone takes the tapes to the set. While the cameras roll in Almería, the music plays back through speakers, and the actors move to it.
The film does not get underscored. The film gets directed by the score.
There is a thing that happens when one element of a piece of work knows what it is before the others do. It exerts a gravity. The actors slow down. The editor finds the cut. The cinematographer holds a hair longer on the close-up because that is the bar where the strings begin to climb. Bronson is said to have adjusted his walk to the harmonica's wail. Fonda's blue-eyed close-up at the end of the massacre had to be timed to a soprano he could already hear in the air. The picture is not being supported. The picture is being told what it actually is.
This is not unique to Morricone. Bernard Herrmann's strings for Psycho — written after Hitchcock had specifically asked for no music in the shower scene — were so authoritative that Hitchcock reversed himself and later credited Herrmann with saving the film. Jonny Greenwood gave Paul Thomas Anderson cues for There Will Be Blood before Anderson had a final cut, and Anderson edited some of his sequences to Greenwood's tempos rather than the other way around. Howard Shore had themes for the Shire and for Mordor in hand before much of Lord of the Rings was even shot, which is one reason the music feels older than the picture. Because it is.
The takeaway is not about who wrote first. The takeaway is about who was willing to let the music tell them what the story was actually about. There is a particular kind of director — and a particular kind of producer, and a particular kind of songwriter — who treats their craft as a question rather than a verdict. They allow another element of the work to know something they do not yet know. They follow it.
This is true at the smallest scale too. A songwriter who finds the bridge before the verse and trusts what the bridge is saying about the rest of the song. A producer who locks in the kick drum sound on day one and lets every other decision answer to it. A singer who improvises one phrase on the demo that turns out to be the thesis of the record, and who never tries to top it on the master. The piece of work that knows what it is, even partially, becomes a kind of compass for everything that has not been decided yet. Following it is not laziness. Following it is humility — the admission that the work has knowledge the maker does not.
Morricone's harmonica was already wailing on the soundstage before Bronson ever lifted it to his lips. By the time the camera rolled, no one in the frame was deciding what the scene was. They were just performing what the music already knew.