John Williams- ET (Extra Terrestrial)
I don't if many people know this, but I saw a documentary about the making of ET's music, and Speilberg liked it so much that during recording for much of the score Speilberg decided to edit the movie to the music. As much as I know that is rarely done, and Speilberg never usually does that. The problem was getting the cues perfect for the movie. John would have to keep editing the music, but Speilberg saw that it took away from the piece so he ended up editing some of the movie to better fit the music. I think that's pretty cool.