During an interaction, the behavior of a person and the presence or absence of feedback by this person affect the behavior of the other participants. A conversation consists of the exchange of meaningful utterances and of behavior. For example the speaker punctuates, reinforces her/his speech by head nods, smiles and so on. Likewise the listener can interact by smiling, vocalizing or shifting her or his gaze to participate in the conversation. Facial expressions and gaze behavior help the interaction because they control the flow of speech and speaking-turn exchange. They depend heavily on the relationship between the participants, their personalities, emotions, attitudes, social identities and so on. All movements made consciously or unconsciously influence the other participants.