Since most of the answers so far have been focused on animation, I'd like to add as a sidenote that the shading model and materials used to render the face can also have a large impact on its realism.
In particular, realistic skin rendering is usually implemented using a subsurface scattering shader, otherwise it tends to look artificial. Reflectance is also important, as skin is slightly glossy and reflects a certain amount of light. Eyes and hair rendering are also important, and both subjects can get extremely complex when pursuing realism.
If possible, use a game engine that already supports these rendering techniques!
I'll just leave a few resources on these subjects below, for those that are interested, taken either from the Cryengine 3 documentation (amazing stuff), or from the free GPU Gems books.
Skin's main look is the result of subsurface scattering (light
bouncing inside the surface, exiting in a different location than
where it entered). Without subsurface scattering skin appears hard
like concrete. It's the most important visual cue for the look of skin
Eyes are very complex not only in shading but also in composition,
therefore their shading/look cannot be achieved via regular shading