The defining shot in Alfred Hitchcock’s Psycho is not any of the images in the famed shower sequence, or the overhead of the private detective getting knifed at the top of stairs or the reveal of “mother” as she’s turned slowly in the swivel chair. It happens at the very beginning, as Hitchcock pans and dissolves across downtown Phoenix, Arizona, before finally settling on the room where Marion Crane (Janet Leigh), a bored real-estate secretary, is having an afternoon tryst with her boyfriend Sam. The camera enters the scene through a crack in the window, under the shades, furtively catching a peep. Hitchcock is the voyeur, and so are we.

Later, when Norman Bates (Anthony Perkins) removes a painting in his office at the Bates Motel and stares through a peephole into Marion’s room, that early shot comes back again. We’re doing more than playing the Peeping Tom now. We’re being drawn into the perspective of a deranged killer, and seeing Marion in a negligee not unlike the one she was wearing in bed with Sam. That’s the trigger that causes the shy, lonely, agonized Norman to take leave of himself before re-emerging for the most celebrated kill scene in horror, and Hitchcock had planted the seed for it about 45 minutes earlier. He even marked the place, time, and date in the titles, with absurd formality: “Phoenix, Arizona. Friday December the Eleventh. Two forty-three pm.”

Hitchcock is acknowledging the sinful allure of cinema itself, which allows us to watch the lives of others without being seen ourselves. He’d done the same thing six years earlier with Rear Window, which turned the apartments across a courtyard into series of screens, with each window featuring its own private drama, played out for the benefit of wheelchair-using photographer with a set of binoculars. But Psycho went a step further, into the male gaze at its most lascivious and predatory. The movies would never be the same again.

Sixty years later, Psycho lands near or atop lists of the greatest horror films ever made, but its influence is everywhere. Entire subgenres – the serial killer movie, the slasher movie, the “sexy thrillers” of 1990s and beyond – would not exist without it, and new standards were set on visceral possibilities of sound and montage, and the way a story could be structured to yank the rug out from under the audience. It was also a lesson in DIY film-making, in what a great artist could accomplish on his own dime, outside the strictures of a major studio and the bounds of public norms and standards. Hitchcock had always understood how films could tease dark and forbidden fantasies out of an audience: as Norman himself says, “We all go a little mad sometimes.”

What must have it been like to see Psycho in 1960, knowing absolutely nothing about it going in? What must it have been like to be that unprepared for the combined shock of losing the main character and having it happen in the most traumatic way possible, with the shrieks and slashes of Bernard Herrmann’s strings and the nauseating chik-chik of a knife penetrating skin? Cinema history clings to the apocryphal story of an audience screaming in fright as a steam locomotive comes toward the camera in the Lumière brothers’ 50-second The Arrival of a Train in 1896, but surely Psycho was a more authentic and lasting trauma. The building blocks for it were present in other Hitchcock films or the black-and-white horror of low-budget Val Lewton productions, but it was nonetheless a leap forward, an incomparable moviegoing experience.

For those of us who have lived with Psycho for decades now, the film holds other mysteries and fascinations, too, especially in the first half, as Marion reckons with an impulsive act of criminality. It would become an ugly feature of slasher films that promiscuous or law-breaking women were the likeliest victims, but Marion both defines the rule and serves as the exception. She runs off with a $40,000 real-estate deposit because she wants to run away and put her own deposit on a more fulfilling adult life elsewhere. She doesn’t arrive at the Bates Motel until 40 minutes into the film, and Hitchcock uses the time to marinate in her feelings of guilt and shame, and her total inability to mask it around strangers. She’s an awful criminal, needlessly drawing the suspicions of a patrolman and a used-car salesman before meeting Norman, who seems to see right through her.

The scene where she and Norman chat in the motel office is the most complex in the film, operating on multiple levels at once. The setting itself is unnerving, cluttered with the birds of prey that Norman taxidermies as a hobby, and it’s obvious to us that he’s mentally unwell – gentle-toned and passive one moment, defensive and sharp the next. But Marion sees a mirror, reflecting her loneliness and regret, and it changes her. “We’re all in our private traps,” Norman tells her, “clamped in them. And none of us can ever get out.” She convinces herself that she can get out, and resolves to drive back to Arizona and return the money, probably without anyone realizing what she’d done. She has redeemed herself in the way that sinful, promiscuous women in future horror films would almost never do, and so there’s a special cruelty in what happens to her. She’ll die misunderstood.

The second half of Psycho initially offers the tantalizing possibility of Norman becoming the main character, as he cleans up after the murder and tries to fend off the inquiries of the private detective Arbogast (Martin Balsam), Sam and Marion’s sister Lila (Vera Miles). (Like Marion, Norman’s guilt makes him a terrible liar – another way their personalities merge.) But the film is looser and more democratic in bringing new characters on to the scene, which gives Hitchcock the opportunity to play our understanding of Norman and his “mother” against the ignorance of those sleuthing on Marion’s behalf. It’s not quite as arresting as the first, but Arbogast’s death and Lila’s dash to the cellar remain white-knuckle moments in a film that might have been all anticlimax after the shower sequence.

The notorious final scene at the courthouse, where a psychiatrist monologues at length about Norman’s personal history and the forces that wrestle inside him has been mocked for untwisting a twisted mind too neatly. And it does feel like a square addendum to a film that’s so full of deviant and morally ambiguous behavior, as if the studio or a ratings board had mandated it. But at the end of all that, we’re left with Norman alone in custody, wrapped in a blanket, fully conquered by his mother’s voice. “Why, she wouldn’t even harm a fly,” she says. The mirthless grin that creeps across his face suggests otherwise.