AI-Mediated Communication (AI-MC) occurs when an AI system operates on behalf of an individual in communication between people. Building on literature in psychology and human-machine communication, the following three studies answered two overarching questions: 1) how do people perceive their own agency when an AI system operates on their behalf in interpersonal communication? and 2) how do people perceive the role(s) of the system under these circumstances? Through a think-aloud study, as well as an online experiment, these studies revealed that people strive to maintain their agency in AI-MC, and devise various strategies to do so. Additionally, even though AI systems can play many roles in a communication task, people view them as social actors instead of as extensions of the self, which impacts how they attribute agency and behave in human-AI interactions. Lastly, people show increased reliance on the AI system when they lack expertise, which underlies decreased feelings of agency as well as changes in collaboration dynamics. Taken together, these studies apply research on agency and human-AI interaction to the nascent study of AI-MC. The results provide insights into the effects of AI-involvement and topical expertise on people’s perceptions and behaviors, as well as how they think about the roles of AI systems.