When a character switches off the last light in the room, a vaguely bluish light slightly dimmer than normal illumination switches on. It's implied that the characters aren't supposed to see what the audience can, which makes sense; a black screen isn't much fun to watch. Funny thing is, most of the time the characters will be unaffected by the supposed darkness, moving about without stumbling over furniture or stubbing toes. In a few cases, a red light will be used instead. For an aversion, when single point light sources cast shadows as they do in real life, see Chiaroscuro.