In a way this kinda goes back to the acting question. Expressions are really tough to get right, and each character is different, which makes it even harder!
One of the best things you could read on this subject is Tracy J. Butler’s Lackadaisy: Notes on Expressions
You may also want to pick up Tom Bancroft’s Creating Characters With Personality
But first and foremost, think about the character’s personality—that shapes the expressions the most. Each character has their own set of expressions.
And one last thing—I wish I could have found the video from the Beauty & the Beast DVD where Glen Keane talks about his process of creating the Beast and his expressions, but I couldn’t find it online. So if you have it on DVD, I highly suggest you watch the behind the scenes stuff. In the meantime you can read this and this on it. :)
Hopefully this will answer some of your questions! Keep practicing and studying.