You should take a look at F.A.C.S., it scrutinises each muscle in the face and the corresponding change in expression: - http://www.cs.cmu.edu/afs/cs/project/face/www/facs.htm (earlier version, includes images) - https://en.wikipedia.org/wiki/Facial_Action_Coding_System The book "Human Anatomy for Artists: The Elements of…
Actually there are still a few different options for laying out edgeloops... Lots of people use the same head mesh and just fit them on top of various sculpts. This is of course for re-usability as you can quickly produce relatively good background characters quickly, and also for some hero characters. But usually you then…