Deception is a relevant issue for the theories of cognition and social interaction. When we deceive, we influence others through manipulating their beliefs. This paper presents a definition of deception and of its functions in terms of a model of cognition and social action. We define as deceptive any act or omission aimed at making others believe something false or not believe something true about the invironment, our identity or our mental states. A typology of ways to deceive is outlined (omission, concealment, falsification, masking, negation, and false confirmation), and applied to deception in facial and bodily expression. An ECA is presented that can simulate, mask, or suppress facial expressions of emotions. The relationship of deception and politeness is investigated theoretically and through analysis of a video corpus. The results of the analysis are used to determine when an ECA masks, suppresses or simulates emotional expression.