Previously, we developed an embodied virtual communication system (EVCOS) for human interaction analysis by synthesis, and confirmed the importance of embodied sharing in avatar-mediated communication. In this paper, we analyze human interaction and communication in interaction-activated conversation for supporting avatar-mediated communication by using EVCOS, and we propose a model for estimating conversational activity on the basis of this analysis. Further, we develop an interaction-activated communication support system for enhancing embodied interaction and communication by applying the proposed model to a virtual audience comprising interactive CG objects. In this system, the virtual audience generates entrained nodding responses as well as dynamic movements based on the estimated conversational activity during the interaction-activated communication period. The effectiveness of the system is demonstrated by means of sensory evaluations and behavioral analysis of 20 pairs of subjects involved in avatar-mediated communication.