Tom Baker as both the Doctor and Meglos. Creates a fascinating contrast when they get locked in a cell together. It also gave Baker the distinction of being the only person to have two waxworks in Madame Tussauds, as the Doctor Who exhibition which opened during production of this story featured sculptures of him as both the Doctor and Meglos.
In a less obvious example, Christopher Owen provides the voice of Meglos' true form, in addition to playing the Earthling on-screen.
Awesome, Dear Boy: The UK press reported that Bill Fraser only took the role of Grugger so he could kick K9.
Creator Backlash: Jacqueline Hill regretted appearing in the story feeling that since she played Barbara, it was inappropriate to appear as a different character. John Nathan-Turner felt similarly, and decided not to bring back former regular cast members as different characters again.
Hypothetical Casting: Co-writer John Flanagan wanted Lee Marvin to play Grugger and was amazed when Bill Fraser was cast as he had not imagined him in the role at all.
Inspiration for the Work: Writers John Flanagan and Andrew McCulloch were struggling to think of the ultimate villain, until they realised that it was sitting there the whole time - a gnarled cactus.
Out of Order: In a possibly unique case in Doctor Who, this one was filmed without the newest regular castmember. "State of Decay", which featured Matthew Waterhouse as Adric, was filmed first, then "Meglos" without him, and then his introduction, "Full Circle". Some wags over the years have suggested it was "so Waterhouse could take acting lessons", but this is just a rather mean urban legend; "State of Decay" was a pre-existing script that was originally intended for Season 15 and abandoned due to Executive Meddling from the BBC, before being resurrected and quickly rewritten to incorporate Romana and Adric instead of Leela, meaning it was simply ready to go in front of the cameras before this story was.
Playing Against Type: Frederick Treves, who was known for playing straight-faced authority figures, hams it up as the villainous Brotadac.
In the original storyline, Lexa simply vanished from the action once she had served her purpose within the narrative. Christopher H. Bidmead thought sacrificing herself would be more dramatic and fitting.
