John Hurt revealed that he declined the role of Caligula when it was first offered to him. Because of the time-span of the production, the fact that Derek Jacobi was the only cast member to appear in every episode and the subsequent commitments of the other cast members, Herbert Wise decided to throw a special pre-production party so the entire cast and crew could meet. Wise invited Hurt to the party hoping he would reconsider. When Hurt met the cast and crew, he was so impressed that he immediately took the part.
The scene in which Tiberius and Drusus "bathe" one another by scraping each other with metal hooks is historically accurate. After exercise, Roman men would cover themselves in a thick oil that acted in a similar manner to shaving cream, adhering to sweat and dirt and lifting it from the surface of the skin. Men would them use a metallic hook called a strigil to scrape the oil from their own or one another's bodies. It would not have been unheard of for athletes- or in Tiberius' case, brothers- to help one another clean after exercise.
Brian Blessed originally auditioned for the role of Tiberius, but was eventually persuaded to play Augustus instead. He recounted Herbert Wise's saying "be as you are, full of flannel", and that he should always play Augustus as an ordinary person, because the reactions of those around him would make him the Emperor. Blessed was further instructed to "think Mafia" and to play Augustus as a genial mob boss like Don Corleone, who can be as friendly and warm as he wanted because his reputation for danger preceded him.
Siân Phillips has spoken about her initial struggle in performing the character of Livia, because she focused more on making the character sympathetic and justifying her motives rather than playing her as straightforwardly evil. "I wasn't achieving anything much... I knew it, and they knew it. They would stand there and look faintly worried." Eventually Herbert Wise told her not to be afraid of playing her camp, saying to "Just be evil. The more evil you are, the funnier it is, and the more terrifying it is."
The series takes place from 24 BC to 54 AD.