When Catherine Schell was hired to play Maya, Barbara Bain didn't want competition. She used her clout to have a clause put into Shell's contract stipulating that she was not allowed to appear onscreen as herself, without her Maya look (including promo material for the show). Schell even had to dye her blond hair red. The original script of this episode had Maya transform into a "human" (the actress' true appearance) that Tony had been looking at in a magazine, then returning to "herself", tearing up the magazine, and storming out of the Medical Center. When Bain read the script, she reminded the producers of the clause in Schell's contract, causing the script to be changed. Gerry Anderson's crew referred to Bain - never to her face - as "Barbara Pain".
Star Wars: Episode IV - A New Hope (1977) had been in pre-production while this season's earliest episodes were being filmed. Andrew Ainsworth of Shepperton Design Studios had built numerous armor and helmets for the film as well as Darth Vader's costume (which unknown to David Prowse at this point would be worn by him). Ainsworth was hired to build the head of the creature for this episode, using a leftover ABS plastic Rebel helmet from that movie as the basis for it.
This episode originally ran short, so new footage was added later, shot during the filming of The Bringers of Wonder: Part 1 (1977), including the opening scene where the Alphans are shown coming down with the sickness (as the original version began with the personnel already incapacitated); the lengthy scene with Tony and Maya in the Weapons Section; and Fraser setting up the electrical barrier.
Zienia Merton was not under contract for season two, so between that and her diminished role in the series, she departed in March 1976 after three appearances. Three months later, Barbara Bain contacted Merton, asking her to return for this episode, which was in the process of being written, since she and Martin Landau would only be available for one day's filming due to a planned vacation. So, Merton decided to return to the series immediately, beginning with Catacombs of the Moon (1976) (after her character had been replaced by "Yasko" for six episodes) on the condition that her character would have a larger role in the series going forward. However, a few months later, Merton was offered a lead role in a Norwegian film (Kosmetikkrevolusjonen (1977), in which she reportedly performed nude scenes). Judging that Space: 1999 would not be picked up for a third season, she accepted the offer, informing Gerry Anderson and Fred Freiberger that The Seance Spectre (1977) would be her final episode before relocating to Norway. During filming of "The Seance Spectre" (in October 1976), the cast and crew would be informed that ITC had canceled the planned 13-episode third season, so Merton had made the right decision. Yasuko Nagazumi then returned for one episode before Alibe Parsons joined the cast for the tail end of the season.
The very next role Guest Star David Prowse played after this was Darth Vader on Star Wars: Episode IV - A New Hope (1977) (offered a choice between that role or Chewbacca, Prowse decided the villain would be more memorable) and would play the role through all three classic Star Wars movies.