By Stephen Robb for BBC News
It's 50 years since Alfred Hitchcock's Psycho was unleashed on a soon-to-be-terrified world. Even if you've never seen the film you've probably been exposed to its extensive influence.
Alfred Hitchcock had made his name as the "master of suspense" with brilliant, glossy thrillers like Rear Window and North by Northwest, but Psycho was altogether different - the like of which most cinema-goers had never seen.
With its shocking bursts of violence and provocative sexual explicitness, Psycho tested the strict censorship boundaries of the day as well as audiences' mettle - and it gave Hitchcock the biggest hit of his career.
Awakened to the box office potential of violence and sex, mainstream filmmakers followed suit. Here is how Psycho changed cinema:
The 45-second shower murder in Psycho is possibly the most famous scene in cinema history.
David Thomson, author of The Moment of Psycho: How Alfred Hitchcock Taught America to Love Murder, has said it still ranks "legitimately among the most violent scenes ever shot for an American film". According to the book Story of the Scene, by Roger Clarke, it "changed cinema forever".
For the film's first three-quarters of an hour the audience has followed Janet Leigh's Marion Crane, building engagement with the film's supposed central character.
Then in an electrifyingly brutal scene, as Marion readies herself for bed with a shower in the decrepit Bates Motel, she is hacked to death by a barely-glimpsed old woman.
THE SHOWER SCENE
Most of the film was shot quickly with a crew from television, but the 70-plus shots for this 45-second scene took a week to film
To get the scene past the censors Hitchcock claimed the knife never touched the victim, but studies have since suggested it does
Chocolate sauce was used for blood, and the hand holding the knife in some shots is Hitchcock's
Janet Leigh said she avoided taking showers for the rest of her life after filming Psycho
Source: Story of the Scene by Roger Clarke
"They [audiences] had never seen anything quite like it before - the total shock of killing off a lead character a third of the way in, and just the complete feeling of disorientation," says Michael Brooke, Screenonline curator at the British Film Institute.
Psycho was filmed in black and white - unusual for Hitchcock by that stage of his career - partly to cut costs, but also to manage the graphicness of this scene. Hitchcock knew he would not get shots of red blood splattering the wall and floor past the censors of the time.
"The shower scene in colour in 1960 would have just been unshowable," says Mr Brooke.
Psycho "opened the floodgates" for screen violence, says Mr Brooke, paving the way for the slow-motion bloodshed of Bonnie and Clyde and The Wild Bunch in the late 60s, up to today's torture porn of Hostel and the Saw films.
Though "watching the film, you think it's a lot more graphically violent than it actually is".
It is a mark of the shift in levels of violence in cinema that Psycho, given an adults-only "X" certificate in the UK in 1960, now carries a relatively tame "15" rating.
SYMPATHY FOR THE KILLER
Arguably the film's most appealing character is Norman Bates, played by Anthony Perkins, the awkward, softly-spoken young man resignedly running the declining family motel and caring for his abusive, invalid mother.
After Leigh's slaughter, the film switches to Bates' point of view and the audience is invited to sympathise with his agonising dilemma over concealing his mother's horrific crime.
"There is that scene just after the shower murder when the car is sinking into the swamp and it pauses momentarily," says Mr Brooke, "and in a weird kind of way you almost want it to carry on sinking. You don't want him to be discovered even though he is covering up this hideous murder."
Mummy's boy - Bates' character was meant to draw viewers' sympathy
The effect was to toy with audiences' sympathies in a way mainstream thrillers hadn't done before.
Psycho is considered the first modern horror film and credited with launching the "slasher" sub-genre. But Paul Duncan, author of The Pocket Essential Alfred Hitchcock, argues its greatest legacy is the shifting point of view that became a common device of the slashers.
Most of Hitchcock's peers worked in the "third person", positioning their camera as a detached, neutral observer of the film's events, says Mr Duncan, whereas Hitchcock's "first person" camera allied his audience inescapably to key characters.
"That, I think, is probably the most copied aspect of Hitchcock's movies," he says.
John Carpenter's 1978 film Halloween, whose considerable debt to Psycho is emphasised by the presence of Leigh's daughter Jamie Lee Curtis in the lead role, is a "very good example of point of view camera being used brilliantly", he adds.
"Don't give away the ending - it's the only one we have! " Psycho promotional slogan
Having switched the film's point of view to Norman, Hitchcock has manoeuvred his audience exactly where he wants them for the film's shattering, shock twist.
"With all the information you are given you believe him to be innocent, and you identify with his crisis," says Mr Duncan.
The film opens on the skyline of Phoenix, Arizona, on a hot December day, the camera panning lazily across the rooftops before casually zooming in through a window to a hotel room where - unmistakably - an underwear-clad Leigh and shirtless John Gavin have made love moments earlier.
RELEASE OF PSYCHO
To keep secret the film's final twist, Psycho was not screened for critics or cinema-owners before release
Unusually for the time, cast and crew had been made to sign non-disclosure agreements preventing them speaking about the film
No-one was allowed into the cinema once the film had started - enforced by uniformed guards, this was an extraordinary measure in an era when people were used to coming and going during theatres' continuous programmes
These first moments are almost a statement of intent from Hitchcock. "He was quite deliberately testing the waters as far as the censors were concerned with the film," says Mr Brooke.
"It took what had previously been only suggestive sexual undercurrents and made them absolutely upfront."
At that time most US studio films were constrained by the puritanical Production Code, which dated from the 1930s and restricted depictions of sex, drug use, drinking, offensive language and anything else that could "lower the moral standards of those who see it".
When he was unable to secure financing for Psycho from studios fearful of the film's potential for controversy, prompting him to put up 60% of a scaled-down budget of less than $1m himself, the director found himself with an opportunity to work outside the restrictions of the studio system and deliver an exploitation film Hitchcock-style.
The overt sexuality of the film's sightings of Leigh in her underwear, the shocking violence - even a shot of a flushing toilet - were radical in commercial cinema at the time.
And while there had usually been varying amounts of humour in Hitchcock's films, it had never before been combined with such dark, violent material as in Psycho. Today, that pioneering blend of shocks and laughs is notably evident in the films of Quentin Tarantino.
USE OF MUSIC
The violins wailing away during Psycho's shower murder scene have achieved the status of cultural shorthand - denoting imminent violent insanity.
Their importance to the impact of that terrifying scene is emphasised by the fact that at pre-release screenings of a cut of the film before the music was added, many viewers reacted with mild indifference.
"It was only with the second version, with the music added, that people just leapt out of their seats - especially when the shrieking violins started," says Mr Brooke.
The most memorable part of Bernard Herrmann's score has now been imitated to the point of being "one of the all-time aural cliches", he adds.
But the entire soundtrack is integral to the mood of the film, from the blast of strings over the opening credits onwards, says Mr Brooke.
Its influence can particularly be seen in films that use music to evoke a sense of menace and heighten sudden shocks, such as Jaws.
This is not the only way in which Steven Spielberg's 1975 blockbuster, in turn massively influential itself, bears the influence of Psycho, says Mr Brooke.
"When they made Jaws, one of the stated aims was they wanted to make America's beaches as empty as American motel showers were when Psycho came out."
Psycho is re-released in the UK on 2 April. A season of films - entitled Psycho: A Classic in Context - runs at the BFI Southbank, central London, throughout April.
Below is a selection of your comments
In 1961 I was an usher at the Paramont Theater in Salem, MA and my job was to hold the crowd back for the last 15 minutes of the movie "Psycho" and we could not tell them anything about the ending.
Paul Duggan, Salem, Massachusetts