This paper presents an evaluation of the believabil- ity, the feeling of co-presence and the game experience induced by a decision model that allows for adaptive and evolutive body interaction between a human and a virtual agent. The theoretical bases are presented and the model is briefly described. We illustrate and discuss an experiment we conducted to evaluate the model applied in the context of a fitness exergame. This evaluation shows that our model can generate an adaptive body behavior for virtual agents comparable to that a human would perform in context where the user has to pay attention to the agent. This evaluation shows also that this behavior improves the perception of the agent in terms of co-presence and game experience. It also shows that the role of the agent in the scenario has an important impact on the human perception of the agent itself.