Predictability may be an important component of cooperative action, or it may arise as a by-product of involuntary entrainment with another’s behavior. Further, differences previously observed in cooperative versus competitive actions may represent a fundamental distinction between behaviors with opposite goals, or they may simply reflect the output of different physical actions. The role of predictability in cooperative versus competitive behavior was directly tested using a joint sequential button-pressing task in which Participant 1 (P1) pressed a key followed by P2 pressing a key. In the cooperative condition, both actors shared the goal of minimizing P2’s response times (RTs). In the competitive condition, P1 tried to maximize P2’s RTs, whereas P2 continued to try to minimize them. It was found that P1 was much more predictable in the timing of his or her presses in the cooperative condition than in the competitive condition, and this coincided with faster P2 responses when cooperating than when competing. A 2nd experiment showed the effects of the predictability of P1’s responses on the speed of P2 responses were similar when P1 was replaced by a schematic hand, showing the responses could not have been due to the transmission of subtle nonverbal cues by P1. These results demonstrate that being predictable is an important strategy in the timing of cooperative joint action, whereas being unpredictable is an important strategy in competition, and that they have opposite effects on a coactor’s ability to respond quickly.