Facial Anatomy Reference - Artnatomya Website & App
I came across this website not so long ago and just had to share it here. This is an incredibly detailed and informative tool for understanding how the anatomy of a human face works. We are very complex organisms and when animating it can be really easy to just ignore what is really going on in the face in an effort to get your character talking and emoting. This tool really breaks down the muscle groups and helps you understand what muscles work together to make you smile, or frown, or anything in between.
I highly recommend you check out www.artnatomia.net - they also have an app that works offline coming soon!