I really love watching the brows on people in both live action movies and in animation. One of my mentors at the online animation school, Animation Mentor described them as the shoulders of the face. Its something that have been much more aware off when doing my planning for a shot. I will do an additional video where I don't worry about the lip sync and just act out the lines in my brows and eyes.
There are 2 films that really stand out to me when I think about great brow animation; Ratatouille (which I think is still one of the most well animated CG films ever made) and more recently with Puss in Boots (which blows my mind animation wise as well), especially the Humpty character as he is all head!
I try to approach the design of my brow shapes based on them being one of 2 categories, Voluntary or Involuntary expressions:
A) Voluntary expressions: This is an expression where the character is choosing to make that expression like when you are judging someone or have a questioning look on your face. They tend to be more complex in shape and are more asymmetrical. They are more controlled in movement and timing wise can be over more frames.
B) Involuntary expressions: Expressions like surprise and disgust that you don't have control over. They tend to be simpler in shape and are more symmetrical. These expressions are faster time wise and sharper in their movement.
Generally though, the shape change is quite fast - brow movements tend to be much sharper than other body parts. They almost always lead the movement of the head/body parts. This way, you will read the change because if it is during the body or head move you will miss it and it also makes the character look like he or she is thinking.
In Pixar's Ratatouille the character Anton Ego really stands out to me for facial animation. He is very reserved in his body movements with tight gestures of the hands and small body weight shifts. However you think of him as quite a cartoony character in that his facial animation is very broad.
This shot of Anton Ego has 6 main brow shapes or "ideas" as seen in the images below. They don't change constantly, but are kept alive and are involved throughout the acting by flexing or squashing within the current shape. This way they keep moving, but they don't change so much that they complicate the performance.
I really like in the first few eyes darts and how the brows support these movements. When he looks left, the outer and middle parts of the brow drop a little as the pupil moves away from the screen right part of the eye.
I love the transition between frames 236 and 249 where not only do you have the brows leading the body/head move, but the inner brows are leading the outer part of the brow movements. So you get that lead and follow. Normally you would not want to do that any more than 1 frame.
But what I like so much about this transition is that rather than going from pose A to pose B there is a breakdown in there. This gives it a bit more overlap and arcing shape in the movement as well. If you had of just gone from pose A to B without the breakdown it would have felt like the the brows were just rotating.
So its important to add breakdowns in the bigger brow shape changes just like you would in a body or head movement.
To finish off the move it has a really nice overshoot and then settles back down into a shape that is not as intense as the overshoot with an ease in which works well as that ease in is there to really as an effect to scare the guy. So its quite a controlled movement.
Puss In Boots
I love this shot for two reasons - the facial animation on Humpty is amazing and.... its got the "ooooooh" cat! Haha!
The brows in this shot are used so well to really support the other movements in the face and too get the sadness and internal struggle across with the little twitches and changes.
Like the Anton Ego shot where his eye darts where supported by small shifts in the brow shape, at the start of this shot Humpty blinks and there is a small compression in the brows to show the skin pulling down over the eyes which really keeps everything connected and adds to that fleshy feel.
At frame 26 you can see the inner brows leading the body movement and expression change where they go down first. The brows then tense up and move down even further at 39 to really help sell that face compression with the mouth and cheeks.
I love the little staggered drops between frames 63 and 80. Adds some texture to the movement, but also helps to show that internal thinking and struggle going on inside the character.
The brows lead again going up at 81 before the head. They lead by 2 frames here before the head comes up on 83-84.
All the little twitches and movements at the end are great as well. He is starting to break down and has a really nice involuntary feel to it.
Anyway... hope this all makes sense! Let me know if you have any questions. And remember, this is just stuff I have observed and found that has worked for me when animating, but I am sure there are many other great ways to do all of these things too.