After you have created facial expressions for your character you can fine-tune the expression by using the Face Key technique. These keys will be overlaid and blended with any already existing facial expressions.
You may first record expressions by using the puppeteering technique.
After you have created the clips, open the timeline panel. The clips will be recorded in the Expression track.
Focus on the Facial Layer track and move to the frame you want to enhance the expression in.
Open up the Face Key panel.
Click and select the muscles you want to use to create an expression with. Drag and pull the muscles in the Face Key window and the facial expression will change instantly.
Fine-tuning the expression using this method will create a new key in the Facial Layer track.
Here is a typical result of fine-tuning a facial feature.
Before
After
Create any facial expressions from using the Puppeteering technique. Then fine-tune them with the Muscle function in the Face Key panel if necessary.