The character of course. I was told by a Disney background painter, who worked on most of the classics, that a BG should look like an empty stage set. Waiting for the character to enter and give a performance. Therefor the BG should look "unfinished" in order to make room for one more element, the animated actor(s).
The first image is a color study by Mel Shaw for The Fox & the Hound.
The Little Whirlwind
Pinocchio
Robin Hood
Sleeping Beauty
101 Dalmatians
