Dawson Casting: The events of the series span 27 years, so some actors go from playing this trope straight to averting it, or from averting it to inverting it, or both. Just to give a few examples:
André Morell, aged 58, played Tiberius from the age of 54 to his death aged 77.
Ralph Bates, aged 27, played Caligula from the age of 18 to his death aged 28.
Freddie Jones, aged 40, played Claudius from the age of 23 to his declaration as Emperor aged 50.
Hey, It's That Guy!: Though not quite as packed with this trope as I, Claudius, it still has its share of cast members who, though not major stars, were and are still familiar from other film and television work.
Among the episodes' title characters, Roland Culver (Augustus) played the Foreign Secretary in Thunderball, Eric Flynn (Germanicus) played Leo Ryan in the Patrick Troughton-era Doctor Who serial "The Wheel in Space", André Morell (Tiberius) played Major Warden's superior Colonel Green in The Bridge on the River Kwai and Professor Bernard Quatermass in the 1958 serial Quatermass and the Pit, Barrie Ingham (Sejanus) played Cabinet member and unwitting mole St. Clair in The Day of the Jackal, Ralph Bates (Caligula) appeared in various Hammer horror films in the 1970s and as the villainous George Warleggan in Poldark, and Freddie Jones (Claudius) went on to play brutal sideshow barker Bytes in The Elephant Man and Thufir Hawat in the 1984 adaptation of Dune.
In the supporting cast, Caroline Blakiston (Agrippina) had previously played Marjorie Ferrar in The Forsyte Saga, Suzan Farmer (Livilla) was in various Hammer horror films in the 1960s including Dracula: Prince of Darkness and Rasputin: The Mad Monk, Donald Eccles (Nerva) appeared as Krasis in the Jon Pertwee-era Doctor Who serial "The Time Monster", Jerome Willis (Macro) played Stevens in "The Green Death" from the following season, Sonia Dresdel (Livia) went on to play the Marchioness of Auld Reekie in the all-star mini-series The Pallisers,note The Pallisers also featured The Caesars alumni Roland Culver and Donald Eccles. Arnold Ridley (Nigrinus) was more famous as Private Godfrey in Dads Army, Gerald Harper (Lucius Vitellius) had previously played the title character in Adam Adamant Lives, and Wanda Ventham (Ennia) may be familiar as the title character's mother (and, off-screen, the mother of the actor who plays him) on Sherlock.
The series even has a few cast members in common with I, Claudius. John Paul, who played Agrippa in I, Claudius, plays treacherous Praetorian Guard captain Cassius Chaerea in The Caesars, while Kevin Stoney played the same character in both series, the seer Thrasyllus.