This study deals with the interpretation of verbal commands for action. After an experimental study of human interpretation of instructions for drawing geometrical figures, we have devised a model whose computerized version is called SIROCO. This model represents an attempt to simulate differents mechanisms implied in interpretation of verbal commands. These mechanisms exploiting contextual informations allow clarifying and completing propositions expressed in natural language. In the model, first, the constraints expressed in present command, in environnement (already present figures), and in background communication, are represented with fuzzy subsets and circumstantial semantic networks (well suited for flexible and dynamical representations). Subsequently, an optimization procedure integrating all this constraints allows finding a relevant response to the command. Finally a simulation which consists in translating instructor's verbal commands in a defined minimal language and making it interpreted by the system shows quite good results for the model.