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 film. The book, based upon O'Donnell's original screenplay rather than this film, was a huge success and spawned a series of best-selling novels until 1996.
Joseph Losey found it difficult to work with Monica Vitti, as she would invariably be accompanied onto the set by director Michelangelo Antonioni, in whose films 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.
Although Dirk Bogarde was very complimentary about Monica Vitti when this film 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 whom he had disliked, saying that she was "beastly".
About a year before this picture premiered, writer Peter O'Donnell wrote a novelization of his draft of this film's screenplay. This novel was called 'Modesty Blaise' and the book was successful both critically and commercially and led O'Donnell to write a series of 'Modesty Blaise' novels alongside the comic-strips that would continue for thirty years.
The 'Modesty Blaise' comic-strip itself makes two appearances of note in this movie. There is a close shot of a painting of the comic strip when the character of Willie Garvin is introduced and in another shot, Modesty Blaise herself is seen looking at copies of the comic strip.
Modesty Blaise is introduced with a close-up of her legs in fishnet stockings, sitting on a couch by two magazines, one showing a color photo of actress Monica Vitti, and on top a full page copy of the Modesty Blaise comic strip where she is depicted as a huntress with a bow-and-arrow and a revolver. Willie Garvin is introduced looking at a blow-up of the same scene, in a wall frame.
The three major villains all seem to want to protect themselves from the Mediterranean sun. Mr. Gabriel carries an umbrella, McWhirter has sunscreen on his nose, and Mrs. Fothergill has a very wide-brimmed hat. Perhaps mean-spiritedly Modesty and Willie later stake Gabriel out in the open sun.
Terence Stamp turned down " Alfie " to make this. His friend and former flatmate, Michael Caine, made the role his own, won his first Oscar nomination, and embarked on a major international career due to its huge success.