This paper defines facial features of "basic" emotions such as happiness or disgust, and then finds ways to combine features from each to get new facial expressions for more nuanced emotions. For example "Surprised + Fearful = Awe".

While it's not the intent of the paper, I'm playing around with using this to model more nuanced emotions on procedurally generated characters and the idea of an "algebra for facial expression" is both aesthetically pleasing and interesting.