Faces can be one of the hardest thing to animate. Any time the face starts looking like a mask, warning lights should go on in your head. The face is flexible, it can squash and stretch. If it's looking stiff try sqashing the cheeks and adding drag to the nose. Or if you can, play around with the shape of the head.
A closed mouth is very important in duologue. If you don't see the closed mouth it's you may need to add an extra frame on it. Always pop out of a “B” sound. Remember you don't hear the “B” until after the jaw opens, not on the jaw closing. “M” is the other way around. It's a humming sound.
The face should always be asymmetrical. A good example of this is the boy in The Incredibles. He never has a symmetrical face. Even a subtle shot should be asymmetrical, even if it's just a little asymmetrical, that little bit goes a long way to make the face look good.
Don't be afraid to move something. Not moving something because your afraid to break the model or the animation you have already done is the biggest crime you can commit as an animator. Feel the character and what he/she is doing. You can always save. I recommend never going back to a previous save. If it doesn't work, its better to try again then to go back. You learn more, and in my experience it looks better anyway.