I know of one Oscar winning composer who couldn t orchestrate. He just didn t seem to understand it, so all of his work had to be orchestrated by somebody else. Now, after the Oscars, he can orchestrate well. Well, when I personally read a script I ll see and hear the movie in my head. I pick up on how the characters feel and of course the action within the screenplay.
Locations dictate certain types of music.
As in the emotions, set up in parentheticals, also give clues to the sound of the scoring. Non verbal indicators are where most of the inspiration comes from. When a writer puts in the script a name of a piece of music, this sometimes helps the composer. Copyrights dictate whether pieces can be used. And sometimes a particular song is already used in too many other movies, so it gets thrown out.