Here are some theories that will be important to you.
Music behind the dialog
Never step on the dialog
When not to use foreground music
You must never play foreground music (loud music) when the actors are talking. This is called "stepping on the dialog".
What happens to loud music
If you step on dialog, the sound mixer guy will happily turn down ALL your music.
Too many notes
Too many notes
King's rule: Don't use too many notes
The king is correct. You can have too many notes when you create a film score.
The most confusing theory of film scoring
Here is the most important theory of motion pictures.
A motion picture conveys the most information in the least amount of time. A motion picture (or television drama) shows a continually changing picture (a new shot every 3 to 5 seconds) and has as many as 100 tracks of sound. All of this the audience eagerly absorbs.
Every available method of conveying information is used in a motion picture.
The same is true for a symphony. It transmits enough information to completely occupy the listener.
The same is true for people who drive cars while talking on the cell phone, eating a hamburger, and drinking a large Pepsi. The driver's mind is so occupied, the driver cannot focus on the road. Not good!
Limit your film score
The rule is your film score must not completely occupy the mind of the viewer of the motion picture. The viewer must also absorb the visual information from the picture.
Your music must not be so full and absorbing that the audience forgets entirely about the rest of the motion picture.
A simple example
If you want to use a piece of a symphony to score a motion picture, remove some of the tracks from the midi score and it will actually sound better. Keep your music thin.