They use temp tracks from the film a lot of time for this . . . because the actual film as not been scored yet. A lot of times - that is true Sometimes they have music written specifically for it - such as Judge Dredd - Jerry Goldsmith wrote the Trailer Score - specifically for that trailer. Though Alan Silvestri would later score the actual film - nothing good came of it :)

Tell me about it. James Horner's scores to Star Trek 2 / Krull / and Battle Beyond The Stars are pretty much all the same score! Huge passages from each film are almost note for note. Which is not to say I don't enjoy his work - it's just that he gets somewhat repetitive in projects. Moreso than other composers I've heard. They'll write progression that they like and do some variations - that's called style. But he has a tendancy to just reuse whole sections.

Another thing I noticed is his score to Honey I Shrunk The Kids borrows heavily from Danny Elfman's score to Pee We's Big Adventure. Listen to them side by side . . .especially the opening tracks.

It's almost as if he's simply tracking a film rather than scoring for it sometimes. (Using stock music instead of creating fresh music) I guess he can, as he's built his own library of stock music :)

It's a shame that Gabriel Yared's score to Troy got replaced with Horner's take on it. While Horner's is a decent score - it doesn't have the emotional resonance that Yared put into his version.

Oh well
-J