Listen to the music playing for you carefully... you can hear, at the beggining two separate themes playing independantly of each other. As the song progresses, the themes start to move closer to eacher other and begin to intertwine. At one point, you can hear the themes overlap and work together. Then they begin to separate again and are not played at the same time at all. This is done to show the amazing duality at play in the personality of Norman Bates. There are other points in the movie where this theme comes back, and it is so subtle, that it is barely noticeable, and yet changes your enitre perspective on the scene.