There's a material from Aaron Blaise about character design, where he talks about the importance of understanding body mass and how it interacts with each other to develop the figure in drawing, and it's super useful for understanding facial expressions. Makes it much more simple to understand how the structure of the face behaves while doing a certain expression.
For example, the mass that we have on the eyebrows area. Notice how when we frown, they contract in the middle of the face and make wrinkles. Or how in an anger expression (grrrr) makes the mass sitting under the eyes and on the laterals of the nose contract with the eyebrows.
It's makes the job of depicting expressions easier when you understand what provokes the structure the behave in a certain way.
Keep up the hard work!
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