Nilabh brings up a good question that is worth quickly clarifying:
"If the body is animated in the first pass and then the Face is done, how do we know, or how do you TIME the face with an already animated body below exactly to the dialogue? isn't there a huge possibility that the lip sync when done in the second pass will not match the body movement which was already done?"
Hi Nilabh! Thanks for coming by and for the question.
This really comes back down to the very first articles I ever wrote for the AM newsletter back in the day (which you can read in the ebook) - there is nothing more important than planning your shot, especially if you're in the first few years of being an animator.
It's the planning that will help you make all of your action and acting decisions, and also your rough timing decisions. Obviously, sometimes you tweak the timing a bit once you have your stuff in the computer, but it should theoretically be pretty close to your original planning unless major changes were required or requested midway through the process.
The planning should help you nail down the timing of your actions, and that includes the facial acting choices -- including eye direction changes, expression changes, blinks, etc. Many animators might not do separate thumbnails for every single eye blink (I don't), but at least make notes to yourself of what frame this expression will happen on, or what frame these blinks will happen on.
The point of planning is to give yourself a relatively fast way to try a lot of different options, settle on your ideas, and make all of your decisions before the decision-making process gets into the computer and becomes a nightmare of editing curves, keys, and controllers. It's so much simpler if you know what you're going to animate before you turn your computer on.
As such, your planning should have already helped you decide when and why your character's face and eyes will be moving/blinking, and how that will make your character's attitude and emotions feel.
If you have time to do some decent planning, then the timing of the face *will* match up to the timing of the body, because you have planned them to work together.
My previous post was all about what aspects of performance are the most important to focus on, but when you really get down to it, it's important to nail them all, and most important to make sure that all the aspects of the body's movement and performance are working together cohesively.
If you've already animated the body, but haven't done planning for the face for some reason (not recommended!) and are under a tight deadline and just have to get *something* finished, then my advice would be to just make sure that you design the facial animation carefully to match the body animation that is already working. If your body performance is already reading, then the face should accentuate it and make those actions, ideas, and emotions even more clear than they already are.
If the face seems to be distracting from the overall actions, ideas, or emotions, then you've definitely done something wrong and need to go back and figure out what's up. Most likely it's a timing problem, or else the facial performance is conflicting with the body language...
Hope that answers your question! Thanks again for swinging by...