It's Very Clearly Stated In The Film:

The Whole Concept Of The Devil Is A Metaphor On One Level.

The Score Is Always The Wonderful Icing. The Score Tells You The Emotional Content Of The Film. What The Characters Don't Say, The Music Can Say.

I'm Not In Front Of The Camera, They Are. I Encourage Them; I Build Up As Much Of Their Confidence And Ego As Possible. They've Got To Take Control; I Can't Act It Out.

When Ray Charles Is Concentrating He's Like A Piece Of Granite, Nothing Twitches, Nothing, ... He Sat For 25 Minutes Solid Like That, Like A Stone, And I Thought, 'oh My God, If He Doesn't Like It I'm Dead.' ... And Then Finally, After 25 Minutes He Started To Talk Back To The Screen. I Heard Him Say 'that's Right. That's The Truth.' .

You'd Be Surprised How Many Movie Stars Still Care About The Work.