The idea of the pins-under-the-eyes torture device came from a joke of Argento's. Argento said it would annoy him when people would look away during the scary scenes in his films. He would jokingly suggest taping pins under people's eyes so they couldn't look away from the film. It would late materialize on the screen for this film.
According to star Urbano Barberini, it would take hours for everyone to re-capture the crows after they were released in the opera house for filming. Around 140 crows were used, but only 60 sum were ever retrieved. The others apparently escaped from the opera house during filming.
Originally, Columbia Pictures intended to release the film theatrically in Germany in 1988 but after the FSK (Official German Censorship) demanded to cut out almost 25 minutes, mostly violence (among others the two scenes with Betty wearing the needles), it was released straight to VHS, even though a theatrical release has already been announced.
During the filming, Ian Charleson got into a minor car accident and was hospitalized for a week where, after having some medical tests done, he was diagnosed being HIV-positive, a condition which he had suspected himself to be for nearly a year prior to filming.
Vanessa Redgrave was attached to appear as Mara Czekova, but dropped out shortly before production began. The role was then reduced to a minor one. According to actor William McNamara, Redgrave was hired for a week of work but exited the film with full salary having never once been called to the set.
There has frequently been controversy about the film's aspect ratio since it was shot in the Super 35 process and released both in 2,35:1 and 1,85:1 on DVD with the general conclusion being that 2,35:1 was the ratio intended by Dario Argento since he had the film shown that way on various festivals and obviously chose to use Super 35 himself for this film.
The trivia items below may give away important plot points.
Daria Nicolodi originally did not want to play the role of Mira. She felt the character was thinly-written and in addition had ended her long-time relationship with Dario Argento two years earlier. What finally convinced her to take the role was the fact that she loved Mira's elaborate and shocking death scene. She would later say that filming her death scene was tremendously scary as it required her to have a small amount of explosive placed on the back of her head.
The film had an alternate ending at one point where after Marco was killed, Betty would realize she was sadistically in love with the killer. The two would then flee together into the Swiss countryside and the film would end.
The ending of the movie is inspired by the ending of Thomas Harris's book "Red Dragon". In an interview in Luca M. Palmerini's book "Spaghetti Nightmares", Dario Argento states that he did not like Michael Mann's Manhunter (1986), the first film adaptation of "Red Dragon" which used a completely different ending from the one in the novel, but is a fan of Harris's novel. (Ted Tally's screenplay for Red Dragon (2002) later adapted this ending more faithfully.)