An amazing reference sheet for animators (or amateur detectives) came across my desk this week:
(click here for the full-size-readable version)
An artist called Cedarseed compiled all of her notes on facial expression to create a "facial map" detailing the differences between emotional expressions! For example: the difference between a "sad expression" and a "blue expression" is that in the blue face the eyebrows are lower and the eyes' irises are partially covered by the eyelids, whereas in the sad face only the pupils touch the eyelids.
Cedarspeed analyzes 60 different expressions in this manner and offers a great starting point for any artist depecting a subtle expression. Of course, facial expressions are not universal (each culture may express the same feeling in different ways) and the accompanying body language can easily change the meaning of an expression. But for a quick facial cheat sheet, they don't get much better than this!