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. The most important is perseverance. The easiest thing to do is always to quit. To give up and say Aw to heck with it, this is too difficult. So many times when something doesn t go the way we think it should or not the way we feel it should be, we look at that as a failure.