In the CSO shots typical of Doctor Who up to this point, two different camera operators were required to accomplish a final shot. One had a camera pointed at a background, while the other had a camera trained on actors. It was laborious in the extreme to ensure that the two cameras were working in harmony. Even the slightest lens movement could make the actors appear to "slide" across the background, or even disappear behind a portion of the model shot. The difficulties of using CSO can plainly be seen in a serial like Underworld, or even in part four of Robot.