It has long been argued that the possibility to interact in and with a virtual environment (VE) enhances the sense of presence. On the basis of a three-component model of presence, we specify this hypothesis and argue that the mental representation of possible actions should especially enhance spatial presence, and to a lesser extent the involvement and realness of a VE. We support this hypothesis in three studies. A correlative study showed that self-reported interaction possibilities correlated significantly with spatial presence, but not with the other two factors. A first experimental study showed that possible self-movement significantly increased spatial presence and realness. A second experimental study showed that even the illusion of interaction, with no actual interaction taking place, significantly increased spatial presence.