Computer simulation of human facial expressions requires an interactive ability to create arbitrary faces and to provide a controlled simulation of expressions on these faces. The system describes in this paper presents the interactive facilities for simulating abstract muscle actions using free form deformations (FFD). The particular muscle action is simulated as the displacement of the control points of the control-unit for an FFD defined on a region of interest. One or several simulated muscle actions constitute a minimum perceptible action (MPA), which is defined as the atomic action unit, similar to action unit (AU) of the facial action coding system (FACS), to build an expression