Expressions and Mirrors

Reference is extremely important when it comes to facial expressions. The easiest reference is yourself, and that is something that animators take full advantage of. Directing someone elses' face is a lot harder, since everyone's faces and ideas of expressions are different. When using yourself as reference, you have much more control over the type of expression you want, and it is easier to explore and find the right one rather than trying to direct someone else. Having a mirror at your desk makes this much easier to, being able to just look over and see it again. Photos are a good alternative though - you an keep them and create a whole library of emotional reference, making it a lot easier to find what you want. There is a risk of using the same sort of reference and expressions each time yo go for a certain emotion though, especially if the collection of expressions is not that big. Using a mirror gives you a sense of spontaneity, and makes sure the artist uses it for more of a loose reference, to tailor to the character than trying to draw that exact face.

How I feel when animating

I find this very good for both animating and drawing, but I find it easy to leave my face in that expressions while doing the actual work, so half of the time I end up with a sore jaw. Well, as a (probably fine) artist said, art is pain.