We open our mouths when we are silent

Lip syncing is an important technology that enhances the impression of embodied conversational agents. However, there is no study to design the mouth movement of the agent when the agent is silent. Therefore, this paper investigated how human speakers move their mouths when silent in dialogues. As a result, we found three facts. First, a speaker does not completely close mouth even when listening to a partner's talk. Second, the degree of mouth opening while talking and listening greatly depends on the speaker. Third, the mouth opening is possibly affected by the next state of the speaker.