Four Flies on Grey Velvet

• Bud Spencer, fresh off a run of Spaghetti Westerns, turns up as the drummer Roberto's beatnik artist pal Godfrey (Diomede in the Italian cut), a character actually nicknamed 'God' who gets introduced to a burst of the Hallelujah Chorus. Argento knew Spencer from his earlier life as a screenwriter, having co-written Westerns the actor starred in, like 1968's Today We Kill... Tomorrow We Die!
• Argento wanted British rock band Deep Purple to score the film, but the deal collapsed. Screenwriter Luigi Cozzi attributed it to an Italian law restricting public arts subsidies on projects using foreign personnel. Ennio Morricone scored it instead, then he and Argento had a falling-out during post-production and didn't work together again until 1996's The Stendhal Syndrome.
• The retina-image gimmick the title hangs on came close to being cut. Argento thought the optography idea (lifting the last thing a corpse saw off its eye) was too far-fetched until effects man Carlo Rambaldi showed him how the shot would actually look, and that changed his mind.
• The Italian box office numbers are documented: from December 23, 1971 to January 30, 1972 the film grossed roughly $1.2 million across 46 theatres in Italy.
• The film spent decades without a legitimate, widely available home-video release and earned a reputation as a lost Argento work. It finally resurfaced for audiences in February 2009 via a remastered Region 1 MYA Communication DVD, which still dropped 30-40 seconds to print damage. A UK Shameless restoration followed in 2012, with full 4K editions coming later.
• The finale's car wreck was shot with a high-speed camera (reportedly a German rig built to film metal stress tests) to stretch the crash and the decapitation payoff into agonizing slow motion before snapping back to real time. The earlier shooting also uses an early bullet-time-style slow-motion effect.

The whole movie is a marriage horror story wearing a giallo coat. The killer isn't some random outside threat but the person closest to Roberto, which reframes every threatening photo and phone call as domestic abuse made literal. The paranoia of being trapped in your own home, disbelieved by everyone, is the real engine, not the whodunit.

The retina device is the thematic key, not just a plot trick. The film is obsessed with seeing and being seen: Roberto is watched, photographed, recorded, while the one piece of evidence that could save him is an unreadable image burned into a dead eye. Argento is arguing that the camera, and by extension cinema, captures everything and explains nothing until it's too late.

Read the killer's pathology as childhood damage detonating in adulthood. The motive traces back to a violent upbringing and identity forced into a shape it never wanted, so the murders become a delayed revenge against a world that mistook the killer for someone else, with Roberto standing in as the unlucky surface that absorbs it.

A rock drummer kills a stalker in self-defense, and a masked figure photographs the whole thing. From there the threats arrive in the mail and the body count climbs around him while nobody believes a word. Argento closes out his Animal Trilogy by trading the cool detective logic of his first two gialli for something sweatier and more paranoid: a man trapped in his own house, gaslit by a killer whose motive curdles out of a marriage rather than a stranger's pathology. The only clue is a dead woman's last image, four dark smudges on grey.