The term “believability” is often used to describe expectations concerning virtual agents. In this paper, we analyze which factors influence the believability of the agent acting as the software assistant. We consider several factors such as embodiment, communicative behavior, and emotional capabilities. We conduct a perceptive study where we analyze the role of plausible and/or appropriate emotional displays in relation to believability. We also investigate how people judge the believability of the agent, and whether it provokes social reactions of humans toward it. Finally, we evaluate the respective impact of embodiment and emotion over believability judgments. The results of our study show that (a) appropriate emotions lead to higher perceived believability, (b) the notion of believability is closely correlated with the two major socio-cognitive variables, namely competence and warmth, and (c) considering an agent as believable can be different from having a human-like attitude toward it. Finally, a primacy of emotion behavior over embodiment while judging believability is also hypothesized from free responses given by the participants of this experiment.