“’How does a film-maker make sure his music is heard? Let me give you an example. If someone has not been invited to a party, but wants to go, what does he do?’ [Ennio] Morricone acts out some noisy Italian bonhomie: ‘Hello everybody, hello.’
‘He doesn’t do this. He knocks at the door, asks for permission to come in, enters the house and then starts meeting people. The music in a film must enter politely, very slowly. The composer does not have to write music at the actual moment a character enters a room - it might be too much. So there is this slow, delicate entry, with a simple sound that allows the film-maker to lower the other, naturalistic sounds.
The human ear can distinguish no more than two sounds of different quality at the same time. Some very nice music doesn’t work because of that: if it is too strong, it can become an element that disturbs the film, rather than giving something to it. Yet in some cases the music must be very, very strong, when it is necessary to give a particular dynamic to the storytelling course of the film, rather than, say, a person’s feelings.”
-excerpted from Morricone profile “Screen Saver”, The Guardian (via)