Deep Red

• Argento didn't hide the killer's face, he dared you to miss it. The murderer's reflection is genuinely visible in a hallway mirror during Marcus's first walk through the apartment, unblurred and on screen, framed so the audience registers it as one of the row of paintings. The whole mystery is fair play; the reveal is just you rewatching the shot you already saw. (Wikipedia; Slant Magazine; horror101withdrac).
• Those black-gloved killer's hands in the close-ups belong to Dario Argento himself. He shot his own hands doing the murders, a habit he carried across multiple films. (IMDb trivia; Wikipedia).
• Goblin weren't the first choice. Argento hired jazz composer Giorgio Gaslini, soured on the work, and brought in the prog band at the last minute. Their leader reportedly turned out two compositions in a single night, and the resulting score launched one of the most important director-composer partnerships in horror. (Wikipedia; ProgArchives).
• Casting David Hemmings as the witness was a deliberate nod to Antonioni's Blow-Up, where Hemmings played a photographer who may have shot a murder by accident. Argento reran the same engine: a man who saw something, can't trust his own perception, and has to re-read the evidence to find the truth. (Slant Magazine; MoMA).
• The mechanical effects, including the giggling puppet that lurches at Professor Giordani and the various severed-head gags, were built by Carlo Rambaldi. His work here caught Dino De Laurentiis's eye for the 1976 King Kong, and Rambaldi went on to win Oscars for Alien and E.T. (Monster Legacy; Wikipedia).
• The English-language theatrical cut runs roughly 22 minutes shorter than the 126-minute Italian version, hacking out gore, the comedy beats, and the romance between Hemmings and Daria Nicolodi, which gutted the actual character chemistry. After Suspiria hit big, Japan even reissued Deep Red as 'Suspiria 2' despite zero connection between the two. (Wikipedia; IMDb trivia).

The film is one long argument that eyewitness memory is worthless. Marcus 'saw' the killer and immediately rewrote it into a painting, and every subsequent death traces back to his refusal to trust the image in his own head. Read this way Deep Red isn't a whodunit, it's about perception failing in real time, with the Blow-Up casting of Hemmings as the tell: the truth was captured and then misfiled by the witness himself.

The childhood-trauma frame makes Carlo, not just his mother, the real wound of the film. The recurring children's lullaby and the crayon drawing of the murder point to a kid who watched his father die and got walled up alongside the secret. Argento stages the killer's psychology as something manufactured in childhood, which is why the nursery tune keeps surfacing before each kill, the past forcing its way into the present.

The mirror reflection works as the film's whole thesis on looking. Argento keeps burying the answer in plain sight, in mirrors, paintings, doorways and reflections, betting the audience scans surfaces without seeing. The theory horror fans chew on is that Deep Red is rigged against the viewer the same way it's rigged against Marcus: you're given everything and trained by genre habit to look past it, making you complicit in missing the killer.

A jazz pianist watches a psychic get butchered through a window, then spends the rest of the night convinced he glimpsed something at the murder scene he can't place. He's right. Buried in his memory is a face he mistook for a painting, and the closer he digs, the faster the bodies stack up around him. Argento turns a whodunit into a trap built from nursery tunes, mechanical dolls, and a single image hiding in plain sight. The clue was on screen the whole time. So was the killer.