We used rapport theory as the framework to examine the effects of LLM-based embodied conversational agent (ECA)’s nonverbal behaviors in the context of health coaching. To conduct this study, we built two types of LLM-based health coaches in virtual reality using the Unreal platform. The less rapport-building health coach displayed minimal nonverbal behavior during conversations (e.g., no direct eye contact, no upper body movement) while the more rapport-building health coach displayed various rapport-building behaviors (e.g., smiling while listening, upper body movement while responding). Participants were randomly assigned to one of the two types of health coaches and completed six coaching sessions with the same health coach in immersive virtual reality (VR). Findings showed that those who interacted with the more rapport-building health coach expressed greater attentiveness across all six sessions (measured via ratio of gaze on the health coach). Also, we found attentiveness and subjective rapport during the initial interactions (sessions 1 and 2) as the most promising predictors of human clients’ overall satisfaction with the intervention at the end of all six sessions. Finally, the results indicated that the more people interacted with the health coach, the more they felt they benefited from the sessions, with effects even more pronounced for those who interacted with the more rapport-building ECA. These findings have significant implications for communication research and practice.
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