Wouldn't it make sense to figure out what kind of expressions humans make by first looking at what muscles they have in their face and then just seeing what all those muscles do?
Dude named Paul Ekman did that, wrote Emotions Revealed (which I recommend). That over exaggerated Lie To Me show is based on him. He developed a system to teach people how to read faces, for police and airline folks and stuff, called FACS-Facial Action Coding System. Best walk thru I can find online is here (and now I'll be able to find it next time) because the real one is pricey.
(nother great resource for learning how the muscles of the face shape the expressions on it.)
looks like Ekman's expanded and shifted his training thing around, FACS & FACE etc. point is, he's the dude that cracked the face