Early on during pre-production, Edgar J. Scherick suggested that Mary Tyler Moore and Valerie Harper could play "Joanna Eberhart" and "Bobbie Markowe" respectively. This was vetoed immediately by William Goldman who deemed it as a "very gimmicky publicity stunt", as both actresses were very popular at the time starring in top-rated sitcoms. The idea was dismissed before casting ever began.
When Joanna goes to the city to show her photos at a gallery, the large black and white photo in the gallery window is of Lewis Carroll, author of Alice in Wonderland. Under his real name (Charles Lutwidge Dodgson), he was a well-known Victorian photographer, especially of children.
When Bobbie Markowe says "... I don't WANT to squeeze the goddamn Charmin!", this is a reference to a television advertisement for Charmin, a brand of toilet paper, which was heavily promoted in the US in the 1970s.
Brian De Palma was approached to direct in early 1974, due to the surprise success of his first feature, Sisters (1973); however, negotiations fell through and he ended up filming the suspense melodrama Obsession (1976) instead.
Bryan Forbes claimed that he found screenwriter William Goldman very high-handed and rude, and reluctant to change anything in his script, which several previous directors had turned down. Goldman, in turn, insisted that Forbes rewrote his script extensively, mostly during shooting.
When the casting process began, producer Edgar J. Scherick, who had secured the rights to Ira Levin's novel desiring to achieve "another Rosemary's Baby (1968)", suggested Mia Farrow for the role of Joanna. The idea was quickly dropped and the actress, then living in England, was never approached.
The trivia items below may give away important plot points.
Katharine Ross had become such a good friend of Paula Prentiss during the shot, that she found the scene where she stabs "Bobbie" very disturbing. She actually got so anxious during takes, that director Bryan Forbes ended up shaving the back of his hand and doing the scene for her instead.
According to screenwriter William Goldman in "Adventures in Screen Writing" (Ch 6), this was the only project he has been involved with that he knew was doomed even before production began. The original concept was that if men were going to murder their wives and replace them with robots, the replacements had better "be in the form of a Playboy Bunny". The concept was tossed when director Bryan Forbes cast his wife Nanette Newman in the film, as per contract. Newman was "an English actress in her mid-40s. An attractive brunette and very talented, but a sex bomb she wasn't". As the result of casting Newman, Forbes and Goldman had an ongoing feud and out went "the parade of Bunnies walking through the A&P in shorts on their perfect tanned legs" and in came the summertime wear of "long dresses to the floor and big-brimmed hats".