This article shows off new technology which allows for extremely realistic facial animation, and I would recommend giving it a look.
The video made me realise that the most realistic animation is achieved when it is captured with the person providing the voice acting. When it is applied afterwards it must be extremely difficult to match up the audio with the visual as they may not be perfectly in sync. I hope that more game developers start using this technique.
Sorry this isn't more in-depth but I'm pressed for time today!