Vision mixer
Vision mixers were regular members of the production crew of the 1963 version of Doctor Who only. They are required by productions using multi-camera setups. Working hand-in-hand with the director, they are essentially "live editors", who controlled when a particular camera was actively recording.
According to early Harnell-era vision mixer,
What I'm doing as a vision mixer is cutting to the camera against the dialogue of the [camera script that the director has prepared]."
Their functionality was highlighted in the documentary, Vision On, from the DVD release of The Sensorites.
Because no mainstream DWU show after Survival has been produced in a multi-camera environment, vision mixers essentially haven't been used since 1989 — with the very brief exception of Trisha Goddard and Barbara Windsor's scenes in Army of Ghosts.
Although Survival was the final story of the regular run of Doctor Who to employ a vision mixer throughout its production, the last BBC-licensed story to use vision mixing was The Curse of Fatal Death. It in fact parodied the work of vision mixers in the original series by showing how quick cutting between cameras created the "endless corridors" down which the Doctor and his companion typically ran.
