In this serial (and much of the following one), Tom Baker has a clear disfigurement of his upper lip. A few days before production began, Baker was bitten by a Jack Russell terrier belonging to Paul Seed (Graff Vynda-K). The makeup team did what they could to cover up the injury with a bit of plaster, but it was still plainly obvious. The problem was explained in-story when the TARDIS lurches, and the Doctor's face bangs into the console.
The planet Ribos was patterned after mediaeval Russia, with the name being an anagram of the common Russian name "Boris". The currency of Ribos, the opek, was a reference to both the Russian kopec (one-hundredth of a ruble) and OPEC, the Organisation of Petroleum-Exporting Countries.
The original plan was to bring back Sarah Jane Smith to help The Doctor find The Key to Time. When Elisabeth Sladen turned it down, Romana was created.
Mary Tamm had trained with Louise Jameson at the Royal Academy of Arts. Jameson played the previous companion Leela who departed at the end of the previous story The Invasion of Time.