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 1968 Doctor Who serial "The Wheel in Space", André Morell (Tiberius) played 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.
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.