There was a study
published recently Nature Neuroscience that shows fear and disgust to be anti-faces. So from a normal neutral face position fear would be a lengthening of the facial features and disgust would be the opposite movement or a closing down of the openings in the eyes and nostrils. The article gets into evolutionary origins of this kind of behavior. But for the standpoint of artistic representation and photo manipulation you can ignore all that and just concentrate on what this tells you about how to warp the facial features to portray or extend the perception of these 2 mirror image emotions.
Of course you could use the various Studio Artist warp tools to help create these kinds of facial emotion manipulation effects from neutral facial photo head shots. Being able to invert a particular warp in the opposite direction seems useful for this kind of thing, so i'm looking into providing an easy way to do that with the new inverse warp paseq context functionality.
One interesting thing to think about is whether the neural perception of these 2 facial emotions literally corresponds to an axis in a neural multi-dimensional facial emotion perceptual space. You could think of this as an analogy to the 2 chrominance axis (red-green, yellow-blue) for color perception. And of course the same analogies for color opponent interaction that can be used in artwork could also apply to these 2 facial emotions being played off of one-another.