Principal photography is the actual filming of the episode. Director, actors, and crew gather at a television studio or on location for filming or videoing a scene. A scene is further divided into shots, which should be planned during pre-production. Depending on scheduling, a scene may be shot in non-sequential order of the story. Conversations may be filmed twice from different camera angles, often using stand-ins, so one actor might perform all their lines in one set of shots, and then the other side of the conversation is filmed from the opposite perspective. To complete a production on time, a second unit may be filming a different scene on another set or location at the same time, using a different set of actors, an assistant director, and a second unit crew. A director of photography supervises the lighting of each shot to ensure consistency.