Great post by Victor Navone on Eyebrows

Animation tip: Have your brow animation precede any head or body movement. Otherwise the brow action will be lost in the movement and the audience will miss it. This technique also helps to make the character look like he's thinking before he's acting.

And here are some interesting behavioral facts about brows:

As the pitch of the voice raises the brows go up

As the pitch of the voice lowers, the brows likewise drop

When asking a question where the answer is already known, the brows raise

When asking a question where the answer is truly unknown, the brows lower

Spontaneous facial expressions (surprise, fear, pain, etc.) tend to be symmetrical, where as expressions we choose to make (curiosity, suspicion, contempt, etc.) can be more asymmetrical.

These are just trends, not rules

For more info please check out the original post