Communicating false information during social interactions places unique cognitive demands on the speaker. One such demand is the increased need to track what another knows to avoid introducing contradictory information. The present study examines the minimal conditions required for vigilant mental state monitoring by using a game-like task that required participants to respond to virtual partners' questions with true or false information. In this task, there were no explicit demands to engage in mental state monitoring. We found increased response times when the answers potentially violated a partner's belief states - but only if participants believed their partner to be an actual cognitive agent. These effects were also shown to be additive to the simple demands in suppressing a truth bias while responding falsely. We argue that participants exert greater cognitive effort as an automatic response elicited by being situated in an interactive, deceptive context.