Sir Dirk Bogarde was cast at last minute. Bogarde originally turned down this movie and Sir Tom Courtenay was cast, then Bogarde changed his mind due to financial reasons - he had taken considerably less than his usual salary for "The Servant" in order to get the film made. As he was in his early 40s at the time, he was rather too old for the role of "Nicholas Whistler", who is clearly supposed to be only in his twenties. Five years later, Tom Courtenay ended up starring in another spy comedy, "Otley (1969)."
The scenes set in Prague, Czechoslovakia were shot in Padua, Italy.
One of five spy movies that Director Ralph Thomas made in just over a decade, the others being "The 39 Steps (1959)" and three consecutive back-to-back-to-back spy movies made during the late 1960s, "Deadlier Than the Male (1967)," "The High Commissioner (1968)," and "Some Girls Do (1969)."
The code number of Nicholas Whistler (Sir Dirk Bogarde) was 008. This movie's American titles "Agent 8¾" and "Agent 008¾" vary this. These titles spoof James Bond's code number of 007. This movie's storyline actually has an Agent 007 who is killed off at the start of the movie.
Sylva Koscina (Vlasta Simoneva) had previously been considered for the role of Tatiana Romanova in the James Bond movie From Russia with Love (1963).