The scene where Jo gets the Doctor lost because she's been holding a road map upside-down was inspired by a real incident when Jon Pertwee and Katy Manning were driving to a location shoot.
Jon Pertwee named this as his favourite Doctor Who (1963) serial. Nicholas Courtney named it as his second favourite.
The documentary being made during this story is for a fictional channel, BBC Three - a channel that now exists on the BBC digital network.
The area under the church is always referred to as "the cavern", never "the crypt". This was a BBC requirement to avoid the risk of causing offence to viewers with religious sensibilities. Similarly, much to director Christopher Barry's amazement, no mention of God was permitted to be made in the story's dialogue, in case this was considered to be blasphemous - although references to the Devil were acceptable.
Due to the extensive amount of film footage on location for this serial, director Christopher Barry decided to use three film cameras which were crystal-locked together so that all three could be started and stopped simultaneously by remote control, similar to the multi-camera studio scenes on video. This was unusual as film footage would normally be achieved with just one camera, but using three enabled more scenes to be captured in the time available.