I would like to disagree with Sean on the this matter.  I find that the liquify tool is the most simple and natural-looking way to make your minifig's mouth move.

To make the liquify effect stick to your minifigure (assuming that it's moving), you'll have to reverse track your footage.  See Andrew Kramer's Demon Face Warp Tutorial (http://www.videocopilot.net/tutorials/demon_face_warp/).  If your animation is a bit choppy, you'll have to keyframe the track points, rather than just click the track button.

Another advantage of using the liquify tool is that you can link your distortion percentage to your audio, eliminating the need to keyframe the width of the mouth!  (http://www.youtube.com/watch?v=hyrGOVzgRMo)  <-Parent the distortion percentage to the "Both" slider on the null object, and add a *x (x being a number of your choice) to the end of the created expression.

I hope this helped.  If you have any questions, feel free to ask me.

I do realize that mouth movement isn't the same as lip syncing, and that the liquify filter only warps the image, and doesn't change it.  It was with this knowledge in mind that I replied to this post.

Lip sync using the liquify filter is impossible.  I'd like to rephrase my previous post and say "The liquify filter is one of the best ways to accomplish mouth animation in After Effects."

And, as I did say before, syncing the movement of the mouth with the audio can be done with no keyframing at all.  You can see how to do this by watching the tutorial at the second link in my previous post.

All good. Apologies for not giving a more detailed explanation, I find myself very short on time at the moment.

Trying to put it succinctly, what I consider to be lip syncing entails changing the mouth shape and details to reflect phonetics. Different mouth shapes and designs might require some to have teeth, others to show the tongue in different positions, some without either, and a combination of these. The Liquify tool wouldn't be of immense help in accomplishing this.

If you want to have a further discussion on it, feel free to hit me up on Skype. My handle is "sean.warton". I can talk and work on my projects at the same time, I just can't always spend time typing replies on forums

All good dude. Just don't get too far ahead of yourself. Master brickfilming as best you can without relying on post production effects (especially for complex things like lip syncing) before jumping in too deep.