It's a simple fact that it takes less time to draw mouths and eyes than it does to draw bodies and limbs. So, by creating a set of happy and sad mouths for lip-synching, it's possible to cut down on the need for time-consuming whole-body animation, while still maintaining convincing character acting. In short: the head does all the work so the body shouldn't need to.
This project gives you an overview of the process I use for this, from creating original pencil sketches through to a finished animation. To accomplish this, it will help if you already have a good understanding of both Flash and Illustrator.
I've included all my original files for you to work through the steps with - although you may choose to follow the process using your own characters if you wish. As there are some fairly big jumps between certain steps, I have also included various work-in-progress files (WIP) for your reference. And it'll be worth looking at the final designer_CA.mov file before you begin, to get an idea of the end results.