When is a vampire film not a vampire film? When it's a touching pre-teen parable set in a Swedish suburb in the 1980s, within a social-realist milieu but revealing the lonely emotional truths of childhood that can reduce the casual viewer to tears. The Swedish smash and festival darling Let the Right One In proves once again that the vampire movie is a malleable and resilient form indeed.

A world away from the anaemic US blockbuster Twilight or the inane British comedy Lesbian Vampire Killers, this film instead describes the sweetly unfolding relationship between a bullied 12-year-old called Oskar (Kåre Hedebrant) and his new best friend, the tough raven-haired Eli (Lina Leandersson). The latter, Oskar eventually learns, is actually a vampire. But the movie doesn't belabour the point. Yes, there are gruesome murders and grisly deaths a plenty, but Let the Right One In prefers to stay with Oskar and Eli, and to linger on the midnight snowflakes that fall on their hands as their fingers, for the first time, gently intertwine.

“I started crying when I first saw that scene,” says the film's 41-year-old writer John Ajvide Lindqvist. “I knew right then, from that one sequence, that we had something special.” At the time Lindqvist and the director Tomas Alfredson were in Lulea, the second most northerly town in Sweden on the Gulf of Bothnia. There were there, in crippling -30C cold, to shoot the movie's many eerie snowbound night scenes. For, thanks to global warming, Lindqvist explains, there wasn't enough snow left in the story's actual location, Blackeberg, a suburb of Stockholm, to replicate life there in the 1980s.

Lindqvist himself is the writer of the screenplay and the bestselling debut novel on which it is based. The idea, he says, began in early 2001 “as an attempt to portray the area that I grew up in, and not as a vampire story. When I eventually brought in the vampire element, I decided that I'd have to write it very seriously, as if it was about a child stricken with a terrible disease that meant that she had to kill other people and drink their blood to survive. But it also meant that she lived a very lonely and disgusting existence.”

The book, an instant bestseller, was sent to Alfredson, who had previously swept the Swedish Oscars - the Guldbagge Awards - with a three-hour black comedy called Four Shades of Brown. The 43-year-old Alfredson immediately recognised some of himself in the bullied Oskar. “It was the story of my childhood, in many senses,” he says. “I had problems, like Oskar, when I was his age. I'd skip school, come home and fantasise that I was the most beautiful guy, the strongest guy, the one that everyone wanted to be with, to f***. The usual stuff.”

Despite the dream team of Lindqvist and Alfredson, bringing Let the Right One In from page to screen was often a horror show. Controversy initially erupted in the Swedish press, where fans of the book feared that it would be “Americanised” and turned into a traditional horror film. Meanwhile financiers flinched at the prospect of funding a movie that had no specific market. “They said: ‘Who is going to watch this?'” Alfredson says. “Is it a film for children or grown-ups? How are we going to market this?' It was going to cost €3 million (£2.8 million) to make, and €2 million is the norm for a Swedish movie. That last €1 million was really hard to raise.”

Then there was the filming itself. Dragging 12-year-old child actors and a full film crew into a near Arctic environment was never going to be easy. “When it's really, really cold, and people are standing around for eight hours in -30C, they start to go crazy, cameras start to freeze and it gets bad.” Alfredson suffered frostbite and lost the use of two fingers on his left hand. “I've ruined them for the rest of my life,” he says, before joking nonchalantly, “but anything for art's sake!”

The children, he says, were easy to direct but were never shown a screenplay. “I would recite the script for them aloud every morning, so that they learnt by ear and not by eye. I tried to keep it as playful as possible.” Nonetheless, he says, they abhorred doing the sweetly romantic climax - a tiny kiss from Eli to Oskar. “They could do such complicated scenes, but being children, they hated kissing each other.”

Alfredson and Lindqvist are, naturally, both sensitive to the possible prurient subtext in the movie, and to the likelihood that the “pervy mac” brigade might be excited by young children, often semi-naked, inserted into the highly sexualised context of a vampire adventure. Lindqvist insists: “I did not want the sexual aspect of vampire mythology coming through here. I wanted them just to be soulmates. It had to be asexual.” Indeed a secondary character, Eli's adult helper Hakan, is a paedophile in the novel but becomes a submissive acolyte in the movie for this very reason. “Paedophilia is such a complicated subject,” Alfredson adds. “You can't just use it as an emotional special effect, you have to take responsibility for it if you bring it up.” He is, however, rightly unapologetic about Eli and Oskar's love scenes and their depiction as characters, semi-clad or otherwise. “Children exist, nudity exists, nude children exist,” he says. “I'm a film-maker, and you have to depict nude children, otherwise you wouldn't be true to your art.”

In the movie's best scenes, often simple night-time set-ups in uninspired suburban apartments, Oskar seems to drip with a quiet infectious innocence that washes utterly over Eli and appears the equal, remarkably, of her primal physical bloodlust. In these moments the movie's depiction of childhood, and its cast-iron belief in the innocence it contains, is deeply moving and certainly unexpected for a genre movie.

Let the Right One In was first screened in Sweden in January 2008. Almost immediately it began to win awards and appear on critics' Top Ten lists. When it scooped the Best Narrative Feature award at Robert De Niro's Tribeca Film Festival, beating the likes of David Mamet's Redbelt and the blockbusting Speed Racer, Alfredson felt that he had finally arrived. “I thought: ‘This has won the prize totally out of our context in Sweden, with people who don't know anything about me, or John!'”

Right now Alfredson, who has subsequently been inundated with offers from Hollywood, and Lindqvist are working again together on adapting the latter's third and as yet uncompleted novel, another supernatural tale, but have no plans for a sequel to Let the Right One In. On reflection, they can't quite articulate why the movie has become such a success. Maybe it's the vampire thing, Alfredson ponders. Maybe it's the thrill of being scared for two hours, and then safe when it's over. No, Lindqvist says, returning to his central thesis, maybe it's love. “For despite what you may think,” he says, “the central story is about accepting love when it comes to you. Even if this love comes from the strangest of places.”

Let the Right One In is released on April 10

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