Director Joseph Losey found it difficult to work with Monica Vitti (Modesty Blaise), as she would invariably be accompanied onto the set by Director Michelangelo Antonioni, in whose movies she had become famous. Antonioni would often whisper suggestions to her, and she would take direction from him rather than Losey. Eventually, Losey asked Antonioni, whom he greatly admired, to keep away from the studios during filming. Antonioni complied.
Peter O'Donnell complains that of his original screenplay, only one line remains: "What do you know about Wilberforce?" However, he accepted the invitation to write a novelization of this movie. The book, based upon O'Donnell's original screenplay rather than this movie, was a huge success and spawned a series of best-selling novels until 1996. [Despite the popularity of this anecdote, one does have to point out, prosaically, that the line quoted does not, in fact, appear in the film].
Although Sir Dirk Bogarde (Gabriel) was very complimentary about Monica Vitti (Modesty Blaise) when this movie was being made, he also claimed, in a radio interview of twenty-five years or so later, that she was the only one of his leading ladies he had disliked, saying that she was "beastly".
Terence Stamp turned down Alfie (1966) to make this movie. His friend and former flatmate, Sir Michael Caine, made the role his own, won his first Oscar nomination, and embarked on a major international career due to its huge success.
About a year before this movie premiered, Peter O'Donnell wrote a novelization of his draft of the screenplay. This novel was called "Modesty Blaise", and the book was successful critically and commercially and led O'Donnell to write a series of "Modesty Blaise" novels alongside the comic strips that continued for thirty years.