Frank Thomas created these different eye shapes. What do they communicate to you? He was always trying to get to the heart of the character. "What is the character thinking and why does he feel that way?" If you can answer those two questions before you ever set a key, in Frank Thomas' opinion, you're well on your way to becoming a great animator.
Contrary to wait you may think, so much of what we communicate to other people doesn't come from out mouths. Body language speaks volumes about what we really feel and think. But even more so, the number one thing that will truly reveal what we think and feel can be found in the eyes. There are social scientists all over the world dissecting the secrets of human communication. What do we tell each other when we don't say a word?
For the film actor, eyes are the most important communicator. They are so expressive and so telling that even if we are watching animated characters, our connection is with the eyes, especially if the goal is for the audience to make an emotional investment in the characters.
You can tell what people's minds are doing when they are thinking or asked to remember something. Here's a fun little exercise you can try with your friends and family. The following is true for most right handed people (reverse all this for left-handed people): As you face them, and their eyes go...
Up and to the right - They are remembering a pictoral image.
Up and to the left - They are constructing a visual image.
To the right - They are remembering sounds or conversations.
To the left - The are constructing sounds or conversations.
Down and to the right - They are experiencing an internal dialogue.
Down and to the left - They are accessing kinesthetic feelings, tastes and smells.