Two experiments tested whether engaging in actions by personal choice vs. external task assignment moderates the effect of incidental affective stimulation on action control (volition). As choice of an action alternative has been found to lead to strong goal commitment, an implemental mindset, and determined task focus, we reasoned that it should shield action control from incidental affective influences. By contrast, external task assignment should lead to weaker action shielding and thus give way to incidental affective influences. Results followed our predictions. When participants were assigned the cognitive task, they persisted less (Study 1) and mobilized lower effort assessed as cardiac pre-ejection period (Study 2) when they were exposed to happy music as compared with sad music. These music effects on volition did not appear among participants who could choose the task. Our results show that working on a task is shielded better from incidental affective influences when the task is chosen rather than assigned.