It has previously been shown that observing an action made by a human, but not by a robot, interferes with executed actions (Kilner, Paulignan, & Blakemore, 2003). Here, we investigated what aspect of human movement causes this interference effect. Subjects made arm movements while observing a video of either a human making an arm movement or a ball moving across the screen. Both human and ball videos contained either biological (minimum jerk) or non-biological (constant velocity) movements. The executed and observed arm movements were either congruent (same direction) or incongruent (tangential direction) with each other. The results showed that observed movements are processed differently according to whether they are made by a human or a ball. For the ball videos, both biological and non-biological incongruent movements interfered with executed arm movements. In contrast, for the human videos, the velocity profile of the movement was the critical factor: only incongruent, biological human movements interfered with executed arm movements. We propose that the interference effect could be due either to the information the brain has about different types of movement stimuli or to the impact of prior experience with different types of form and motion.