This special issue describes a number of applications that utilize lifelike characters that teach indirectly, by playing some role in a social interaction with a user. The design of such systems reflects a compromise between competing, sometimes unarticulated demands: they must realistically exhibit the behaviors and characteristics of their role, they must facilitate the desired learning, and they must work within the limitations of current technology, and there is little theoretical or empirical guidance on the impact of these compromises on learning. Our perspective on this problem is shaped by our interest in the role of emotion and emotional behaviors in such forms of learning. In recent years, there has been an explosion of interest in the role of emotion in the design of virtual humans. The techniques and motivations underlying these various efforts can seem, from an outsider's perspective, as bewildering and multifaceted as the concept of emotion itself is generally accus...