I did extensive facial animation, taking into account mouth, jaw and head motion, and facial muscles movement. I observed my face, as well as the actions of the characters, trying to be as accurate as possible. I noticed that the lower left corner of the lips was characteristically lowered and the upper right corner was exaggeratedly elevated, so I created suitable blend shapes and used them frequently throughout the animation.
I continued adding blendshapes using a soft brush, thinking about the facial movements and solving emerging problems. The hammer skin weights tool was very helpful in adjusting single wrong points.