View Sticky Lips
Sticky Lips is an animation and rigging technique that mimic the tendency of human lips to adhere to each other a bit as the jaw is opening (typically for speech.) You may notice this yourself if you have not been speaking for awhile, and then gradually open your mouth; you will feel your lips almost “unzip” gradually. Adding this capability in an animator-friendly form to the facial animation rig will add a great deal of realism to your facial animations.
As with almost any different rigging challenge, there are many different ways to implement a “sticky lips” rig. Almost every approach will involve a combination of morph targets and bone rig. Some scripting and/or expresso work can help to automate the rig as needed.
Do not be put off by what would seem to be a difficult task; like anything else, a solution will present itself as you work through the problems. Most of the “exotic” techniques and solutions that production companies like to put in their “behind the scenes” articles or video additions are really based upon what you already know: modeling, morphing, skinning, expressions to drive controllers, and so on. In fact, simpler solutions tend to perform much better than exotic simulations that require a bunch of coding. In the end, it’s all about what looks good (and what the animators can/will use).