Until Friday 22 July this year, Norway was a country of 5 million people that, even if tried, could never make the headlines. It was a land of stunning scenery, tasty cinnamon buns, slightly rubbery cheese, expensive beer, depressing literature and terrible pop music; a place where, once in a while, someone might cause a fuss by stealing Edvard Munch's iconic painting The Scream from Oslo's National Gallery. But when a rightwing fanatic began his murderous rampage, Norway's image lost a lot of that innocence.

As the country struggles to come to terms with its tragedy, an unlikely ambassador for the real heart and soul of Norway is nearing the UK. Entitled Troll Hunter, this lovely, mockumentary-style, comic fantasy brings the area's centuries-old folklore into the modern day, as a team of student film-makers, investigating a series of illegal bear killings, encounter a mysterious-looking hunter (Otto Jespersen) with a bizarre story.

The film is the brainchild of André Øvredal, a thirtysomething former film student and commercials director, and it's a testament to the film's charm that, when we meet, on the third day of a long weekend in London, not one person has so far seen fit to mention the murders.

"This is actually the first conversation I've had about that," he says, with genuine surprise. "Of course, it was devastating. Just horrendous. We couldn't believe it. But the two things are so unrelated, I can't really put them together. The only thing is, if people have a certain image of Norway now, maybe the movie will have a positive impact. Overall, I think Norway hasn't actually changed much. Some things may, but I don't think the general feeling of being there will change. We consider that day to be a complete fluke." He shakes his head. "It doesn't reflect anything."

'I wanted to change a lot of the details around the troll mythology, or rather add some mythology of my own'

Glen Erland Tosterud in Troll Hunter.

A child of the Spielberg-era blockbuster, Øvredal cites Ghostbusters and Indiana Jones as major influences, but the real starting point for Troll Hunter is the 1992 Belgian film Man Bites Dog, a low-budget satirical mock-doc in which a movie crew gets involved with a criminal. That part seems simple to understand, but why trolls?

"We've had the trolls since the Viking age, so I think it's part of our being, practically," Øvredal shrugs. "And there were all these stories, stories that have been told orally throughout the centuries, that were collected in the mid-1800s and put into a book. And when they put out the book they also added some beautiful drawings, and these drawings inspired me almost more than the fairytales themselves. Because the drawings are really ominous. They're very dark and frightening. They don't look like a joke at all; they look like monsters."

Øvredal began by putting all this detail into a script, which he wrote in three weeks at the beginning of 2008. Did he make sketches? "No," he says. "I left all that to others, but I guided them by describing the trolls in the script and drawing from the way Norwegians expect trolls to look. I wanted to change a lot of the details around the troll mythology, or rather add some mythology to it. But the one thing I felt I couldn't touch was the way they actually look. So they are quite close to the way trolls look in that original book. We had artists to do all that and I would sit with them, saying, 'Yeah, that's a good one … Can you tweak that one? Make him bigger, arms longer … ' There was another guy making them in clay. Then we went to the effects houses, and they started making digital versions of our drawings and models."

The startling thing about the finished film is that, though they look physical enough, the trolls are entirely computer-generated. Aside from the cost, Øvredal says, there is always an issue with time: "When you're doing something digitally, you know right away if it's working. If it's not, you scrap it and re-render it. But if you're building something, it can take two weeks for you to realise you've made a mistake." In the case of Troll Hunter, this allows for an astonishing variety of trolls, each with their own distinctive look and personality. "I think the final troll, at the end of the movie, is the one that is for sure the most trollish," says Øvredal. "Also, it's so extremely well rendered by the effects house. But I also love the three-headed troll, because it's just so … nuts!"

He laughs: "In Norway there have never been 'types' of troll. This type of mythology – you know, the different species of trolls – is my invention. I was trying to make an animal kingdom out of the trolls, in a way. Because in the fairytales, they wear clothes and they talk, and there's more of a human interaction. They're basically beastly humans. I wanted it to be the other way round; I wanted them to be more like animals, with a human touch."

'Initially they wanted me to write and direct the Hollywood remake but I felt like I'd end up competing with myself'

Otto Jespersen in Troll Hunter.

The film's advertising campaign began quietly in Norway last year, first by slipping out innocuous documentary footage, then following that with shadowy YouTube clips of the trolls. But when the full trailer appeared, the cat was out of the bag, not least because its star, Otto Jespersen, is a household name over there. "Otto is basically the most famous comedian in Norway," says Øvredal. "He's very controversial."

Troll Hunter has been shown at film festivals around the world, and is now set to be followed by the inevitable "compliment" – an American remake. Øvredal turned down, he says, some 30 or 40 offers, before selling the rights to Home Alone director Christopher Columbus. "Initially they wanted me to write and direct it, but …" He sighs: "I felt like I was going to end up competing with myself, trying to outdo myself. I'd rather do something else."

Øvredal isn't at all concerned with the ethics of remaking, but does seem pleased that the makers of the new Troll Hunter will be taking the production back to Norway. Meanwhile, Øvredal is reading genre scripts of all kinds – "Anything," he says, "from horror to sci-fi and adventure" – as he prepares to make his Hollywood debut. Is that something he always aspired to? "I won't deny it, that's for sure," he grins. "I studied in Santa Barbara and lived in LA afterwards, trying to 'make it', if you will, with my student projects. And I didn't. Then my visa ran out, and I had to go back to Norway. So then I started doing commercials. At that time, I didn't like LA too much, because I was on the outside, trying to figure out a way in. So I promised myself I would never go back to LA unless I was invited back. Which happened!"

Looking at Troll Hunter, with its dark humour and craft, and it's hard not to think of another film-maker who started out much like this: New Zealand's Peter Jackson. Øvredal modestly bats away the comparison but does concede that he's become something of a minor celebrity in his homeland. "Somehow, yes," he smiles. "But perhaps my name is better known than my face. I don't really ever do anything outside of making films, so even if I was given the opportunity to become a real celebrity I wouldn't want it. But to promote my films?"

He weighs it up: "Hmm, I guess that would be fine."