Interaction of music and film
This is my view (and I take the perspective from both sides since I've both directed and composed, also it's apart from the technical aspects of getting the length cues and so on):
The music and sounds should breathe with the film, as a wholeness, become an entity, not just be added as a glaze or ornament afterwards.
As a director I would discuss the movie and listen to some ideas or maybe even drafts from the composer early in the process, and tell the composer if it was in line with my intention.
(Luckily, for me, when I have been the director of a few films or theater plays, the composer has been me, and I have understood the intentions of me as a director
.) As a composer working with some other, I would try to remember these aspects, try to respect and grip the director’s vision.
The composer should let the director lead, and try to perceive the underlying emotional intent, but with a good composer the director should be open to the possibility that the music, even a single tone, could add some important value to a scene that could affect how it should/could be cut.
(It could perhaps just be about adding half a second and let an actor’s emotion linger.) With great scores, the movie and the music gets inseparable.
The composing is a creative process, give it time to grow and evolve, let the composer visit the set, meet the characters, let them plant a seed in the composer’s mind.
Maybe I feel this way just because I mostly like a certain kind of movies where music is central.
Hitchcock and Herrmann probably, I'm not sure, worked a bit like this, and I really like those movies and Herrmann’s music. Don't know about Williams/Spielberg but think of Close Encounters, the music is interwoven in the story.
I'm not saying it's impossible to get a good score after a film is all shot and edited. (And of course it all depends on what kind of persons the director and composer are.)
I earlier made some examples, just for fun, on how music can influence images. I just made some very basic but different tunes to a looped spaceship animation. It’s not exactly what the discussion is about here, but it’s a little related. Even a piece of music that is ever so slightly different (wrong or just off) is going to have great impact on the movie. Have a look if you want to. (Last example has a little more twist at the end.)
https://sites.google.com/site/mixxusstudio/film-tv-score-composition