Kate O'Mara successfully screentested for the role of Annabelle Hurst, and was then offered it by producer Monty Berman. According to her memoirs, however, the American backers refused to cast her after describing her as too "exotic". She did, at least, guest as Pietra in Who Plays the Dummy? (1969).
It is noteworthy that although the Department's Headquarters are in Paris, France, and Stewart Sullivan and Annabelle Hurst apparently have an apartment there, they have a British car with U.K. registration numbers. Whereas Jason King, the English author who seems to live in the U.K. at least some of the time, has Swiss number plates on his (likewise British) car.
During filming lulls at ABC Elstree studios in Borehamwood, Joel Fabiani would often visit the My Partner the Ghost (1969) set, finding their atmosphere far more laid back than on his own series.
There was reportedly a good deal of tension between Peter Wyngarde and Rosemary Nicols during production, suggesting that they hardly exchanged a word outside of filming.
Director Cyril Frankel recalled that the production team's first choice for Jason King was Kenneth More.