Film debut of actress Lara Wendel.
George Hilton was cast in My Dear Killer by Tonino Valerii. Valerii stated that the role was "difficult" to Hilton and that he was "told by many people that it does not suit you". Hilton considered the role to be a "challenging task" as he found Valerii a "very demanding director." This included the final scene where Hilton has a long monologue, that was changed only allowing Hilton half an hour to memorize the new dialogue. Hilton spoke positively about the film later, saying that of all the films he had done he would save about four of them, including Massacre Time, The Ruthless Four, The Case of the Scorpion's Tail and My Dear Killer.
The only Giallo directed by Tonino Valerii, a filmmaker better known for numerous Spaghetti Westerns. He pays tribute to his roots in this film by having a character watch DJANGO on TV during a scene.
Jose Gutierrez Maesso is credited as a screenwriter on the film, but did not actually contribute to the script of My Dear Killer. He was credited for co-production reasons. .
Tonino Leoni and Roberto Valerii reshaped the script, with particular attention given to the psychological examination of the characters. For one thing, they added the naked little girl that appears at the door of the artist uncle to allude to the fact that he might be a pedophile. They also added more depth to Stefania's mother, who sinks into madness after the loss of her girl.