I tweeted earlier about how listening to the Dark Knight soundtrack makes even mundane tasks like tidying your wardrobe feel slightly heroic. And that reminded me of a comment I half-remembered from George Lucas.
In this interview, George Lucas comments on how he believes that audio is 50% of a movie's impact, and a good soundtrack gives you the most bang for your buck in terms of impact.
Given that he worked on Star Wars I can see why he thinks that. But it's probably true, if you ever watch a scene (esp. the threatening ones) and consider what's actually happening onscreen vs. the way the music makes you feel, you'll realise what an effect it has.
I was searching for a source for the quote when I came across this interview with John Williams about the Star Wars soundtracks. The most interesting part for me were his comments on how an orchestral score appealing to the traditional nature of the stories was an important factor in its popularity, his thoughts on George Lucas' modifications to the original films, and a nice story about how the cantina band tune came about. Worth a look.