  This paper presents a study of the dynamic coupling between a user and a
virtual character during body interaction. Coupling is directly linked with
other dimensions, such as co-presence, engagement, and believability, and was
measured in an experiment that allowed users to describe their subjective
feelings about those dimensions of interest. The experiment was based on a
theatrical game involving the imitation of slow upper-body movements and the
proposal of new movements by the user and virtual agent. The agent's behaviour
varied in autonomy: the agent could limit itself to imitating the user's
movements only, initiate new movements, or combine both behaviours. After the
game, each participant completed a questionnaire regarding their engagement in
the interaction, their subjective feeling about the co-presence of the agent,
etc. Based on four main dimensions of interest, we tested several hypotheses
against our experimental results, which are discussed here.
