The interaction between the various layers of the face generates the complexity of facial deformations. Therefore, it is difficult to isolate representation techniques from deformation techniques: each model is an association between a geometric representation and some deformation tools. In some cases, as with the finite element method, the geometry of the model is strongly linked to the deformation method.
On one hand, techniques depend on the desired application. For example, the requirements for medical applications may be drastically different from the requirements for animation. On the other hand, it is often desirable to get as complete a simulation as possible of the entire structure (bones, muscles, skin, and internal actions leading to deformation are important). In certain cases, the visual effect (deformation of the external layer) is all that matters and the issues may be computation time and manipulation tools.