Soundtracks are an integral part of a movie. Many movies just use current pop songs or older classics, but as I am sure you know a large segment have a composer write a score. Aside from people liking music, and thus the reason to put it in movies, music provides an emotional underpinning for a scene. Say I would like a chase scene to be more thrilling, we add some music with a tempo that is on the quicker side, generally music that is thrilling, tempestuous, etc. Watch the car chase scene from Quantum of Solace. The music really starts at around 1:00.
Now, the music from James Bond in this case is largely based on themes written by composers from earlier movies (who doesn’t recognize the Bond trumpet flare?). But most movies and their composers do not have this luxury. Many movie composers, even the most famous ones have to write themes from scratch, a task they don’t always do. This isn’t necessarily wrong, imitation is the sincerest form of flattery, and their is nothing illegal about using themes that are in the public domain. Take John Williams, one composer who has probably profited to most from lifting emotive patterns from predecessors.
Star Wars’ imperial theme could be straight from Holst’s Mars of The Planets:
John Williams is not the only famous movie composer to mimic the orchestration and colors of an earlier composer. Here we have Howard Shore in Lord of the Rings imitating Orff’s Carmina Burana. Listen to the following clip to where Sauron has waded into the battle field (2:55).
The great thing about both these composer’s works is that they are able to achieve the same affect as the work that we would label as “generative”, while using new themes and motifs. Then again, they have wholly sacrificed using their giant soapbox to create new interesting art music and introduce it to the largest audience any classical composer will get.
Okay, this is really not pertinent to your post, but hilarious:
http://www.youtube.com/watch?v=1KaOV3dBlts