I was interested in the way Copland described the expressive part of music because when working on a piece I typically rely heavily on the "musical" aspect of a piece or the things notated in a score to find the expression of a piece that I often don't really think of them as separate. I've always thought of the sensuous or physical experience of music as being the result of expression written into a piece and a performer's take on it so it was a little odd to try and separate those concepts. That being said I think it's really useful when analyzing music to take a step back and make sure you're accounting for both the strictly technical aspects of music and the more abstract feeling concepts of music so that you can get a more complete picture of a piece.