Robert is best known as an actor, but is also a writer and director. A superb mimic, he has often done voice-overs to replace actors who have died or lost their voices. He did the voiceover for Tim Moxon as John Strangways, a British agent in the first James Bond film, Dr. No, then continued to dub actors in James Bond flims such as Adolfo Celi as Emilio Largo in Thunderball, Tetsuro Tamba as Tiger Tanaka in You Only Live Twice and John Hollis as Ernst Stavro Blofeld in For Your Eyes Only. He also appeared onscreen in the Bond film On Her Majesty's Secret Service.
Robert also provided the voice of the mysterious "Number Two" during the opening credits of several episodes of The Prisoner. Normally, the guest actor playing Number Two would appear in the opening credit sequence reciting some of its signature dialogue; in episodes where Number Two's identity was not revealed until the end, Rietty's voiceover was used.
He has been awarded the title of Officer-Knight of the Italian Republic. He is editor of the drama quarterly GAMBIT. He was nominated for a BAFTA special award in 1993. He has translated the entire dramatic works of Luigi Pirandello into English and also popularised the dramatist's work in the anglophone world.