When people perform joint actions together, task knowledge is sometimes distributed asymmetrically such that one person has information that another person lacks. In such situations, inter¬¬personal action coordination can be achieved if the know¬ledgeable person modulates basic parameters of her goal-directed actions in a way that provides relevant infor¬mation to the less knowledgeable partner. We investigated whether systematic violations of predicted movement duration provide a sufficient basis for such communication. Results of a joint movement task show that knowledgeable partners spontaneously and systematically violated the pre¬dictions of Fitts’ law in order to communicate if their partners could not see their movements. Unknowing partners had a benefit from these violations and more so if the violations provided a good signal-to-noise ratio. Together, our findings suggest that generating and perceiving systematic deviations from the predicted duration of a goal-directed action can enable non-conventionalized forms of communication during joint action.