In this video, I create a rig and using Geometry Nodes to make a transparent mask inside the lip gap, revealing the inner mouth.
There are two meshes: a head with Geometry Nodes applied and a separate mesh for the lips (including the inner mouth). To create the transparent mask, I assign a vertex group around the inner lip area and for head mesh I assign a Principled BSDF and a Transparent BSDF shader. Then, Geometry Nodes calculates where the transparent mask should be applied based on the lip opening. As a result, when the lips open, the inner mouth becomes visible.