DR Ron Pulliam, here's SS himself on this:
***SPOILERS***[/size]
"It's an open secret that the music for Sweeney is an hommage to a film composer named Bernard Herrmann. When I was fifteen I saw a movie called Hangover Square, an American movie which took place in London, starring Laird Cregar as – wait for it – an insane composer who was musically way ahead of his time in 1900. Every time he would hear a high-pitched note, his mind would go blank and he'd go out and murder the nearest person. He doesn't know what he does in these blackouts, and he goes to George Sanders, who is the police psychiatrist, who says, “Dear fellow you're in a lot of trouble”. He's arrested right in the middle of playing his piano concerto, but insists on finishing the concert, and sets the entire concert hall on fire. Everyone leaves and he's left playing surrounded by flames. Bernard Herrmann wrote the concerto in such a way that it ends with solo piano with low chords. It's a brilliant score, a one-movement piano concerto. When I was fifteen I sat through the movie twice because in the middle there's a three-second shot of the score on the piano. I memorised it and I can still play it. As a matter of fact, when I visited the Library of Congress, they showed me a number of Bernard Herrmann manuscripts, including Citizen Kane, and they said that they're going to try and get me a photostat of the score for Hangover Square.
Herrmann had a way of making suspense lushly musical, and he had a harmonic line which I thought was just right for Sweeney. I didn't consciously copy him, but it was Hangover Square that started that kind of thought process in my head.
The reason there's so much music in Sweeney is that I thought every time the music stops the audience will remember they're in a theatre and that these events are ridiculous. That's what good film scoring does. I remember when I saw Jaws, the moment those double basses started, and all you saw was water…. I was so frightened. I didn't know what was going to happen, all I knew was I was scared to death. Music can do that to you and it can sustain suspense even when nothing is going on on stage. Herrmann's harmonic language is always unresolved, so something is always going to happen. There's constant motion but the harmony is never quite resolved."