Think of music as an additional actor perhaps playing a variety of characters or if suits you better, it can be a single character displaying a variety of emotions and moods.
On-scene or off-scene
As an actor, they may or may not be on stage all the time. It most films, there will be times when music is not present. When there are characters talking on screen, then it's likely that music will be subdued or absent. When a character is moving from one scene to another, music may tag along. If there's a scene change or location scene-setting, then music is more likely to be present. It may be whispering, but the melodies and rhythms will be present.
Music as the narrator
My preferred way to think of music is as the narrator. Think about a movie with a narrator, you could potentially replace the narrator with music. Often there is music supporting the narrator as well - double narration. With music as narrator, themes can be introduced, moods conveyed or scenes set.
By thinking of music as narrator, you'll ensure that there is space for the music
An article in the Lessons Learnt series from Award Sounds