For location shooting, K-9 was equipped with a small microphone and speaker, so that John Leeson could hear and interact with the other actors from a van parked nearby. One day, while waiting for a scene to be set-up, Tom Baker sat down near K-9 and began working on the Times crossword (Leeson's hobby), conversing with Leeson through the remote link. Without thinking about it, Leeson remained in character. He was later told that several young fans, who had come by to watch the production, were quite amazed by what appeared to be the Doctor and K-9 working on the puzzle together.
The interior sets for the hyperspace ship were made of wood. Normally, to avoid having the sound of the actors' footsteps sound like wooden thumping, would have required foleying (re-recording the footsteps on a different surface during post-production). In this case, however, the sound engineer put microphones under the floor and then electronically altered the sound so that the footsteps sounded (appropriately) like walking on a metal floor.
John Leeson (K-9) moonlighted at the time by taking photos of fellow actors for "Spotlight", an annual actors' casting directory. During production of "The Stones of Blood", he took several photos of Beatrix Lehmann (Professor Rumford), which she was quite pleased with. As a token of appreciation, she gave him a 1936 Leica camera that she had received from Sir Ralph Richardson. As of 2007, he still has the camera.
The K-9 prop was notoriously difficult to operate, especially over rough terrain. In one scene, when the Doctor tells K-9 to go find Romana, the robot turns and then apparently rolls at fairly high speed across a plowed field. In reality, the robot rolls up a small ramp onto a trolley which is hidden in the grass, and then pulled along via cable.
David Fisher drew heavily upon British mythology for the names he used in his serial. Vivien Fay and her past alias, Lady Morgana Montcalm, recalled Morgan le Fay, King Arthur's sorceress half-sister. The word "Cailleach" itself was a Gaelic term meaning "old woman", and was associated with numerous entities in Celtic mythology. Dr Thomas Borlase was an homage to two celebrated historians, Thomas Price and William Borlase. The Megara, on the other hand, were named for Megaera, one of the Furies (also called the Erinyes or Eumenides) of Greco-Roman mythology; these were goddesses who persecuted those they perceived as guilty of a terrible crime.
David Fisher envisioned the Ogri as rocky-skinned humanoids who looked like regular stones only when stationary. This was deemed too costly, although elements of Fisher's original idea - such as the large footprint found by the Doctor and Romana - remained in the script.
The script originally called for the Megara to appear as floating metal balls. Over fears that this would seem to be a rip-off of the training seeker in Star Wars: Episode IV - A New Hope (1977), they were redesigned using a combination of electronic puppetry and optical effects.
The exterior of the mansion and estate shown in the story were actually a business management school that was near the Rollright stone circle used for filming. One night, the TARDIS prop disappeared -- it had been trucked into the nearby village by students from the school.
This was the 100th Doctor Who story, occurring in the 15th season. When it turned out after editing that the first episode was slightly short, director Darrol Blake tried to create a brief "birthday" scene, in which Romana would present the Doctor with a cake, and K-9 would sing "Happy Birthday" off-key. The BBC got wind of this, nixed the idea, and instead inserted a small "review" scene about the Key to Time story arc to fill the time.
The use of video to capture the location scenes instead of 16mm film was a directorial decision by Darrol Blake, who hated the clash between the picture quality of film and video that was commonplace on other stories.
The trivia item below may give away important plot points.
When Romana is pushed off the cliff, the scene was originally filmed with 'Tom Baker' (I) pushing her (this was intended to be the Cessair disguised as the Doctor). Over fears that young viewers wouldn't understand why the "Doctor" would do this, the scene was later re-edited so that the attacker isn't seen. However, Romana's dialogue makes it clear that she thought she was pushed by the Doctor.