Mesh head animators need to remember that often many parts of the face are altered when one part animates. For instance, when you smile, it's not just your eyes that may need to animate: your nose may widen and be pulled down or to the side, if you do a quirky smile. Your ears may also move.
Sadly, not everything can be adjusted with an animation. Photographers should also use the avi's shape editor when needed. For instance, when you smile, your mouth moves up and your lips are stretched and appear thinner. Your cheeks also move out and up. Your mouth also moves closer to your teeth, but there is no way to edit this.