Another way of working is setting deliberate constraints that aren't musical ones - like saying, "Well, this piece is going to be three minutes and nineteen seconds long and it's going to have changes here, here and here, and there's going to be a convolution of events here, and there's going to be a very fast rhythm here with a very slow moving part over the top of it." Those are the sort of visual ideas that I can draw out on graph paper. I've done a lot of film music this way.