Both things happen.
Yup, and sometimes the latter can end up screwing the composer, I.E. Alex North’s discarded score to Kubrick’s 2001. Kubrick was too attached to his temp score and North was never going to match or top that.
I bought that CD score at a Tower Records. A few years later, I was pleasantly flabbergasted to observe North reusing his 2001 end theme for a key moment in GOOD MORNING VIETNAM!
North immediately reused his 2001 score for another MGM movie released in 1968, The Shoes of the Fisherman. It’s a great score, and I quite like the film, too.I bought that CD score at a Tower Records. A few years later, I was pleasantly flabbergasted to observe North reusing his 2001 end theme for a key moment in GOOD MORNING VIETNAM!
Amusingly that was never one of Cameron's complaints. The fights were about Horner not being fast enough and not responding to Cameron's (frequent) edits of the film quickly enough.
Lalo Schifrin's AMITYVILLE HORROR score was literally thrown outside into the trash years before when William Friedkin rejected it for his EXORCIST. Another sidebar: for those who have EXORCIST's original album, very little of it's in the theatrical cuts.
In the end he got an Oscar nomination. Lord knows AVATAR has similar musical strands of Horner's past hits, including TITANIC at times.
The first time I heard Horner's music used in two of the major Marines combat scenes, I thought ''That sounds a bit like a Vietnam film.'' Maybe Cameron suggested that to him originally. Digression endeth.