- Blooper: At the end of the story, you can just about hear a hungover Matthew Waterhouse throwing up at the back of the set while the Doctor and Tegan are discussing the plot.
- The Cast Showoff: The Fourth Doctor's regeneration into the Fifth had the conceit that the Doctor was deliriously thinking he was still in his own past regenerations, allowing Peter Davison the opportunity to show off his gift for impressions as he played the first four Doctors.
- Hostility on the Set: Of a joking variety in an outtake from the filming sessions, Janet Fielding can be seen shaking her fist and jokingly yelling "you wrote this!" at Christopher H. Bidmead, the story's author, after multiple takes with her and Sarah Sutton having to carry heavy stacks of books up the Castrovalva set's stairs.
- Mistaken for Servant: Location shooting was done at the estate of Lord de la Warr. The story goes that on the crew's arrival at the site the producer, John Nathan-Turner, found his lordship amongst the flower beds and assumed he was the gardener.
- Orphaned Reference: The oblique references to something sinister deep within the TARDIS were intended as foreshadowing for an ultimately unproduced story entitled "The Enemy Within" by Christopher Priest, which would have aired later in Season 19 and been about a monster that lived in the centre of the ship.
- The Other Darrin: Director Fiona Cumming cast different actors for the security guards chasing the companions at the start of Episode One than Peter Grimwade had used at the end of "Logopolis".
- Out of Order: The increased lead time between seasons meant that this was the first season shot substantially out of order. This serial was the fourth shot, after "Four To Doomsday", "The Visitation" and "Kinda". John Nathan-Turner claimed that this was to give Davison time to acclimatise to the role before shooting his debut story, but the truth is that the Fifth Doctor's originally intended debut story, "Project Zeta-Sigma", had been abandoned as unworkable, leading to stories set later getting pushed forward in the production schedule whilst a new story was found.
- Real Life Writes the Hairstyle: Peter Davison's hair is light brown at the end of "Logopolis" and the beginning of this story, when he first appears as the newly regenerated Doctor, but has changed to near platinum blond as he is helped through the field to the TARDIS (though this could be explained as being a post-regenerative side effect).
- Throw It In:
- Matthew Waterhouse was severely hungover and vomiting behind a tree in the scene at the end of Part 4 where the Doctor and Nyssa talk about landing the TARDIS. Peter Davison and Sarah Sutton kept acting, and the producers kept the shot, deciding it was useful anyway: it would show the ill effects on Adric's health from being under the Master's control.
- The Doctor impersonating his past selves was added when Peter Davison did it as part of his preparation.
- Wag the Director:
- Peter Davison requested that his character start being credited as "The Doctor" from Season 19 onwards, rather than the traditional "Dr. Who"/"Doctor Who", as the older credit was incorrect in the context of the show's canon. This change would be upheld all the way until the 1996 TV movie, before switching back to the credit of "Doctor Who" in the revival series. This would again be changed to "The Doctor" once David Tennant took on the role, at his request due to "Doctor Who" being canonically incorrect.
- In the screenplay for Episode Two, Tegan was scripted to take a drink from the stream they were near while Nyssa attempted to repair the wheelchair. In the 2018 making of documentary for the story's Blu-Ray release, Janet Fielding revealed that she refused to shoot that scene as scripted at all her father was a parasitologist, and she knew full well what could be lurking in river water.
- What Could Have Been:
- Richard Griffiths was considered for the part of the Fifth Doctor before Peter Davison was cast. Griffiths was later considered as one of the production team's top candidates to play the Eighth Doctor had the series not been cancelled in 1989.
- Elisabeth Sladen and Louise Jameson were each asked to reprise their roles as Sarah Jane Smith and Leela respectively in order to ease an audience into a new Doctor. However, both of them turned down the offer due to scheduling commitments to other projects.
- Working Title: The Visitor.
- Written-In Infirmity: Adric appears slightly ropey at the end of the story, not because of the ordeal he suffered at the hands of the Master, but because Matthew Waterhouse had too much to drink the night before and filmed those scenes while suffering from a hangover.
- You Look Familiar: Michael Sheard appears as Mergrave in his fifth role on the series.
Trivia / Doctor Who S19 E1 "Castrovalva"