Our eyes are the window to the sole, telling much about character. Glen Keane would advise, if you are going to make an error, do not make it on the eyes. However, what about the lower face? It certainly feels to have lesser importance when animating, but can learning the muscles help decision making regarding shapes and controls? I'm not sure, but that's no excuse not to study.
A friend tonight sent me a link regarding the facial action coding system. Click here. As a simple starting point, my plan is to go through the list of action units and action descriptors and see what I can find out.
Starting with the orbicularis oris, which, when contracted, pushes the lips together. Note that the lips are pushed together and out.
Valerie Martin 's youtube collection).
Pulling from either side, is the zygomaticus major muscle. This allows us to smile.
Notice how the risorius pulls the corners of the mouth sideways.
*to be continued