music shouldn't steal the show
Though often memorable and interesting, with famous composers like Bernard Herrmann, John Williams and Carl Davis making their reputations in the genre, film music is almost invariably designed to accompany what is seen on screen. It is not usually intended to be listened to in its own right, and care has to be taken that the music doesn't draw attention to itself by a mismatch with what is on screen. (The exception would be a deliberately-intended counterpoint, where the music is at odds with the visuals for a particular dramatic or comic effect).
Exceptions include the musical, where it is often the narrative which provides a context for musical numbers and spectacle. It is also true that, with film music sales and merchandising representing an increasingly important revenue source for films, music can often play a more defining role.
Of course, music is frequently taken from another context rather than being composed for the film: the Blue Danube waltz in 2001: A Space Odyssey (Stanley Kubrick 1968), for example, or Iggy Pop's Lust For Life in Trainspotting (Danny Boyle 1995). Here, some of the music's pre-existing associations or meanings are recruited to serve the narrative, while the movie serves to re-market the music!