Sheelagh Wells had to concoct a special type of make-up that would crack and crumble on cue. This was more difficult than it sounds, especially when studio lighting conditions are taken into account. However she prevailed and the Ultra's face crumbled right on cue.
Sheelagh Wells asked for her onscreen credit to be removed from this serial after disagreements with costume designer Nicholas Rocker. Despite her protests, she was presented with costumes that both dislodged the Ultras' neck veins, and rubbed off her blue skin paint. As a result, she asked for her name to be taken off the episode as Make-Up Artist.
Three bald actors were required for the Ultras. One was willing to shave his head, while another was reluctant.
Michael Keating was suggested for the role of Vila after appearing in the Doctor Who (1963) serial, The Sun Makers. The tunnels in this episode are the same ones featured in The Sun Makers: Camden Town Deep Shelter, London, NW1.