The complex eye sketch I posted yesterday proved to be difficult to animate. So, I've redone that sketch in Photoshop and Adobe Flash, first to simplify, then to change all the shapes into vectors for easier animation.
Flat colored surfaces with distinct boundaries make for objects that are easier to animate.
What eye movements can be animated? Pupils move left-right and up-down. Eyelids expand to cover the entire eyeball. The eyes can be closed and narrowed. Eye movements change the perspective of the structures to follow facial movements.
Here's my finished sketch from yesterday. I didn't add much since I posted it.
The objects within this sketch were too heavily textured for easy animation. I'll worry about animating textured objects later in the game.
I'm going to do the animation of the eye in Adobe Flash... only because that's what I'm accustomed to. I'll move into a real character animation program later.