At 1:48 in this video, http://vimeo.com/36068222, this animator has the character's face moving horizontally by animating a mask. Looks pretty easy to do.
What I'm wondering, is whether it's possible to have a character animated in this style move their head both horizontally and vertically at the same time. So, that would be two expressions sliders (vertical position and horizontal position) and 6-9 base shapes (up and center, center and center, down and center, up and turned, center and turned, down and turned, and maybe a few pictures in between). But to do that, After Effects would need to be able to interpolate between multiple mask/shape paths at once.
I doubt that AE has this feature built into it (though if it did, that'd be awesome), so I'm wondering if that can be done with expressions. For example, I know it's possible to write an expression that will interpolate a layer's position with the following code: