In a post on the Classical Music & Opera Forums, a member wrote:
Regarding film scores: if they garner less respect (in general) [as music], it's because the external constraints are so much more severe, so that purely musical considerations -- especially considerations of structure -- have to take the back seat (if they're even allowed on the bus at all). To begin with, the composer has to fit the music to a scene precisely x.y seconds long, and if the music is to illustrate the action, then the emphases (beats, accents, phrase climaxes) have to fall precisely in time with visual cues within those x.y seconds. Not much room for sonata form here!
We elaborated a bit on that by pointing out that it's not so much a question of structure or form, but rather a question of narrative.
Every piece of stand-alone music traces out, from beginning to end, it's own perceptible, coherent musical narrative absent which what's written is gibberish, not music. But unless a composer has a collaborative arrangement with the filmmaker such as that between Eisenstein and Prokofiev, in Nevsky most especially, where the film, from its very inception — shot by shot, even frame-second by frame-second — was created at the same time the music was being written and vice versa, a film composer simply cannot think in those terms. The controlling narrative is the film's narrative always, and the film composer, who typically doesn't enter the creative process until the film is in its finished, final-cut form, must work his music to precisely fit that film narrative which leaves him all but powerless to create music with its own, stand-alone musical narrative. In fact, to the extent the film composer writes music with its own, stand-alone musical narrative, to that same extent will that music fail as music for the film for which it's being written.
The hallmark of a first-rate film composer is that his film music is never experienced as a thing in itself unless one consciously turns one's ear to hear it in that way, but instead is experienced as an inseparable and organic part of the very fabric of the film itself. Bernard Herrmann had a particular genius for this, and his film scores have never been equaled much less surpassed by any other film composer of our experience. (Kubrick's brilliant use of already written, stand-alone music in his films is a prominent exception to this rule. But only a Kubrick could pull off that little trick so effectively and make that stand-alone music seem an inseparable, organic part of the very fabric of the film itself.)