Here's another fish tale from David Lynch's Catching the Big Fish:
Sound
Sometimes you hear a piece of music, and it marries to a scene in the script. When I'm shooting, I will often play that piece of music in the headphones while listening to the dialogue. Hearing the music is just a verification that things are going the right way--for instance, the right pace or lighting. It's just another tool to ensure that you're following that original idea and being true to it. So it's a good thing if you've got some music up front to play to see if the scene works.
Sound is so important to the feel of a film. To get the right presence for a room, the right feel from the outside, or the right-sounding dialogue is like playing a musical instrument. You have to do a lot of experimenting to get that just right. It usually happens after the film is cut. But I'm always trying to gather what I call "fire wood." So I have piles of things I can go to and see if they'll work. You just have to pop one sound in, and you realize right away, Oh, that is not working
