Below is an excerpt from an interview with famed film composer Bernard Herrman and Alfred Hitchcock talking about the music in Psycho:
Herrmann (voice over): And, ah, when the music was recorded and we were dubbing the film and we got to the murder scene and we ran the scenes without the music and then I suggested to Hitch that I would like to show him the same scenes with music. And he said, "I thought we agreed not to have any." And I said, "We can have it that way, but at least listen to it the way I feel about it, that way I have written it." And he said immediately [after seeing the scored version], "We must have the music, of course!" And I said, "But you were against it." And he said, "Oh, no. All I made was a poor suggestion."