As we get into facial animation in class, one thing that is often overlooked is the brows. The brow is such an important signal for telling you, the audience, how a character feels. So what?, you might say... What do I really need to know about the brow? Well, here are few things that you want to keep in mind when animating them...
For one, the brows should be treated as a unit. You often see animation notes that show the invisible line that connects the eye brows. This is done to retain a sense of anatomy and design. You don't want two French frys sitting up there. Another thing you want to try to incorporate is how you design the poses. An example of this would be leading the eye. If a character is looking screen left, you really want to pose the brows in a way that complements the direction of the look. It seems basic, but it is missing in a lot of work.
Transitions. The timing between brow poses is important to get right. You really want to think about how the actual muscle works. When I see a brow drifting for a lot of frames, it just doesn't look right. Usually, the transition between poses is relatively quick. Also, think about the clearest "change" from one pose to another. Don't over complicate it. The most successful brow animation I see is well designed, has good timing and is subtle as well. You can get really fancy with how your brows emote. Think of all the small details that a face has. Understand the difference between a brow going up and down, as opposed to Left and Right. Each has different meanings.
Complement them with the eyes. Usually when the brows are moving, you will get some small movement in the eye lids. This is important to understand. There is no rule for how much, its just something to be aware of. I can go into detail about this, but observation is your best friend here.