If you create 2d cutout characters or use toon shading to create a 2d look, this small presentation from Adam Parker might interest you. Adam shows his discoveries in creating a view-dependent face rig in Blender so that you are able to get the best possible look to your character based on your animation poses.
Adam shows how to employ blend shapes or shape keys to modify the face rig based on how it is positioned in world space. Things might look a little strange in the viewport, but once the face-rig is rendered with a 2D look, everything becomes clear.
There is quite a bit of info out there on setting up character and face rigs in Blender, some of which have been posted here before. Daniel Kreuter walked us through the basics for creating a face rig in Blender with Shape Keys and Drivers, while 3D Artist Karan Shah took a more 2d approach by covering how to rig a 2D cut out character.
