The movie is based on "Diabolik," one of the longest running - and most successful - Italian comic strips (known as "fumetti"). It was created by Angela Giussani and Luciana Giussani, two Milan sisters who built a small but very profitable publishing empire out of the King of Terror's success. In the comic version, "Diabolik" is much more sinister than its cinematic counterpart - he's a criminal fighting evil with evil, often resorting to murder to "punish" the evildoers he meets. The film was made assuming some knowledge of the fumetti, thus explaining the negative reaction it initially received outside Italy, although it has since been reevaluated as a classic of 1960s cinematic psychedelia and pop art.
This film was produced by Dino De Laurentiis (later responsible for the infamously mammoth King Kong (1976) and L'Ouragan (1979)), and its 3-million budget was much more than director Mario Bava was used to and enabled him to work with more money and a much more prestigious cast than he normally did. However, he remained true to his principles, relying on imagination rather than money, and brought in the film substantially under budget, at a mere $400,000. De Laurentiis was so impressed with Bava's saving him so much money that he offered Bava the opportunity to make a sequel with the leftover funds, but Bava had by then tired of working with the producer and decided to pass.
The American trailers feature a brief shot of the exterior of Eva's white Jaguar, in which Eva can be seen wearing a white long-sleeved dress that bears no resemblance to the sleeveless red dress in the scene featured in the film. This is the only remnant of footage featuring Catherine Deneuve as Eva known to exist.
When Valmont is trying to determine Eva Kant's appearance, the "identikit" sequence generates images of Barbarella (portrayed by Jane Fonda in Barbarella (1968), the sister production of this film) and British model Twiggy. The resulting image of Eva was drawn by Angela Giussani and Luciana Giussani, with their usual design of the character being modified to resemble Marisa Mell.
John Phillip Law was asked to audition for Diabolik by Dino De Laurentiis as a favor because the filming of Barbarella (1968), for which Law had already been cast in as Pygar, had been postponed. An avid comic book fan since childhood, Law studied Angela Giussani and Luciana Giussani's original "fumetti" for inspiration, as he had done for Barbarella. Upon realizing that the most distinctive aspect of Diabolik's physical appearance was his eyebrows, he applied mascara to his own and trained himself to convey a wide array of expressions with them. Upon meeting with De Laurentiis and Mario Bava, the director exclaimed "Ah, questo Diabolik!" ("This is Diabolik!"), indicating to Law that he had won the part.