It is one of the most notorious scenes ever filmed – yet Hitchcock and Janet Leigh didn't tell the truth about it. Will Hodgkinson on a real-life story of body doubles and murder

In the run-up to the release of Psycho in 1960, Alfred Hitchcock did everything he could to build up the suspense. "No one will be admitted to the theatre after the start of each performance," declared the poster, bearing a sulky-looking Hitchcock wagging a finger. The director bought up all copies of the original novel, which he had optioned for a paltry $9,000, so that hardly anyone would know how the story ended. He also filmed on a closed set and forced cast and crew to sign an agreement promising not to mention the ending to anyone. There were no advance screenings.

When the reviews for Psycho, which is rereleased this week, rolled in, they focused on one shocking moment: the shower sequence, in which Janet Leigh is slashed to death. Comprising over 70 shots, each lasting two or three seconds, it has become one of the most infamous moments in horror movie history. Mixing fast cutting and Bernard Herrmann's screeching music, Hitchcock created a brilliant illusion of gore, violence and nudity – while actually showing very little.

The greatest illusion, however, was to give a very clear suggestion that it was Leigh being hacked to death, by Anthony Perkins as a cross-dressing maniac. Leigh, in her first interview after the film's release, shared the audience's horror: "I believed that knife went into me. It was that real, that horrifying. I could feel it!" In later interviews, Hitchcock and Leigh categorically stated that it was her body in the shower scene – but it wasn't. The body belonged to a model called Marli Renfro. When you can't see Leigh's face in the shots, you're looking at her body double.

A Dallas-born stripper who worked in Las Vegas, Renfro was one of the first Playboy Bunnies. Apart from Psycho, she only appeared in one other film, Francis Ford Coppola's 1962 soft-porn comedy-western Tonight for Sure. Then she disappeared, forgotten – until a news report in 2001 said a 34-year-old handyman had been sentenced for raping and strangling her, a crime that had occurred in 1988 but had gone unsolved for over a decade.

The US writer Robert Graysmith – author of Zodiac, the classic account of the 1970s San Francisco serial killer – was fascinated. He had been interested in Renfro and had always thought about writing a book about her. Now it seemed she had died in a murder that was a gruesome echo of the fictional one she had helped to make so famous.

The original 2001 Associated Press report said Kenneth Dean Hunt, the handyman, had been convicted of "killing two women, including an actress who was a body double for Janet Leigh in the film Psycho". This actress was called Myra Davis; subsequent press reports explained that this was Renfro's real name.

Graysmith probed deeper into the story, certain that something didn't add up. In December 2007, he read an interview with Davis's granddaughter in which she expressed confusion at the shower connection. "My grandmother would never have done any nude work," she said.

Graysmith made two discoveries. First, that Renfro and Davis were two separate people; and second, that Renfro was still alive. The confusion had arisen from the fact that, while Renfro was Leigh's body double on Psycho, Davis was her stand-in, used to check lighting set-ups. In his new book, The Girl in Alfred Hitchcock's Shower, Graysmith suggests that Kenneth Dean Hunt was a Psycho obsessive who wanted to kill Leigh's body double but got the stand-in by mistake. "Everyone confused them," he says, "even a murderer. I discovered Marli was still alive. It turns out she's been so busy fishing in Utah, hiking in Alaska, swimming with dolphins in Florida and generally living life to the full that she had no idea she was meant to be dead."

Renfro, who now lives in California's Mojave desert, only found out about her supposed murder when Graysmith told her about it. "She wasn't a vain person, and had no interest in her past career," he says. "She didn't read articles about herself; and, after her first husband made her burn all her old glamour photographs out of jealousy, she didn't keep a scrapbook."

This might also help explain why she never asked for more than her initial $500 for essentially starring in the most famous movie scene in history, in a film that made $15m in its first year alone. "Janet Leigh went around telling everyone how embarrassed she was filming the shower scene, and Hitchcock backed up the story," says Graysmith. "They concocted a lie."

Tony Curtis, Leigh's husband at the time, claimed in his autobiography that Psycho's success, and the fact that all anyone wanted to talk to her about was the shower scene, drove his wife to drink, which eventually led to her breakdown and their divorce. Had she and Hitchcock been more open about how the shower scene had been achieved, it might not have become the subject of such speculation and obsession. Fifty years after Psycho's release, Graysmith hopes his book will finally bring this all to an end.

The Girl In Alfred Hitchcock's Shower, by Robert Graysmith, is published by Titan. Psycho is rereleased on Friday.