One thing I find missing with facial animations often is blood flow under the skin. Skin often looks plastic because it doesn't change hue with pressure changes

One thing I find missing with facial animations often is blood flow under the skin. Skin often looks plastic because it doesn't change hue with pressure changes. An example of this would be firmly pressing your lips together - they'll go a shade lighter due to the pressure causing the blood to flow out of the lips. These things are almost imperceptible, but they go a long way to making something look alive.