First they started building a subway system, all around Stockholm, and after they built that they built the suburbs around it, so it was very Swedish/German planning. The intentions were good, but when people moved in it became something else. So in the beginning, in 1958/59, this housing project Blackeberg, where the book is set and where we shot, was ready, it was a very cute place, but in the late 70s, early 80s, when the film takes place, it was a nightmare. 

But it was very beneficial for a filmmaker, because everything looks the same. It has the same colour palette. And you have a town in miniature, with this square in the middle and the houses all around it. You get a miniature landscape there.

So it was a bad area?

Yes. In those days, over 20 years ago, there was a lot of drugs and social problems there. When I was young it was considered a dangerous place.  The best way to see the difference between Sweden then and Sweden now is to look at old pictures of that town square. 

There used to be a pharmacy, a liquor store, the social security office, a library and a Co-op.  Today, there's a solarium, a tattooist and a video porn shop! Which paints a very revealing picture of what has happened to Sweden in those 20 years. Because, really, we were living halfway behind the Iron Curtain then.

Was it important to set the film in 1982?

It's not a very important thing in the film, but for some reason it's easier to tell this kind of story in the past tense. It's also interesting to look back. But I didn't want it to be nostalgic, because that's a very easy way to score points. So it's quite subtle about nostalgia.

It's obviously filmed in some incredibly cold locations. Was that a worry, going in?

Yes, it was a big worry, because, aesthetically, it's very hard to have all this white snow, because it makes white holes on the film negative it just looks blank! So we had to find a way to portray snow without losing any of its texture. The darkness helped a lot, of course, to establish the soft light we have in the film.

At night, the snow isn't really white, it's grey. It's really like velvet. But to get the right texture you have to work in very low temperatures. You have to go down to, like, 20 or 30 degrees minus to get some effect, with the icy breath and all the frost and everything, so that was very tough to do, working in those temperatures. Because the cameras kept stopping, and the kids were very thinly clad. It was very complicated. But it looks very nice!

How did you cast the children?

It was almost impossible. They get old in six months, so you can't judge by looking at their headshots, as you do with adults. We had to hold open castings. We spent a year trying to find them and we went all over Sweden to look for those two. The most complicated thing, I thought, was that it wasn't just about finding the right Oskar and the right Eli.

Because I consider them to be the same character they're two sides of the same character so they had to be their own mirrors, not just in terms of looks but also personality. It was really tough to make that choice.

What did you tell them? Did they understand the film?

I told them what this film was about, briefly. But you can't give children the responsibility of making a complete portrait, from A to Z. It's too much to carry. To a grown-up actor you can say, 'This is a neurotic mother who becomes blah blah blah in the end,' and the actor can take responsibility for that they can create a portrait with different stages and turning points. But you can't do that with a child.

I didn't want them to read the script because I thought it would give them a lot of mental images that would be impossible to get rid of, especially if they were wrong. So I told them the story, very briefly, and then they got the script each day from me, reading it out loud.

So they learned by ear, not by eye. That was a good method, I thought. Kids age very quickly on a movie set, so I really didn't want them to turn into young adults too soon. We also kept them out of the publicity around the film, so they have not given any interviews.

Why not?

They're kids. They should stay kids, I think.

The film suggests at certain points that Eli may not be real. Is that a possible reading?

Yes, that's a possible reading. You could even read it that Oskar dies in the swimming pool at the end, and that the train scene at the very end is him going to heaven, or something. I have a more romantic view on it. I think Oskar also becomes a vampire and that they work together. (Laughs) These things are open in the book, too.

Why do you think Sweden lends itself so well to this kind of story?

Well, we obviously have a lot of darkness here in the winter. It's noon now and it'll be getting dark in two hours or something. It's really depressing; December and January are really heavy. No wonder people drink!

Are there Swedish vampire myths?

No, there aren't. But there are a lot of stories about dangerous wolves!

How do you see the vampirism in this movie? Is it a metaphor?

In this story, it's a symbol for Oskar's anger. Because, most often, bullied children are portrayed as very sad people, but I think they're very angry, and this anger can't come out. I would think that bullied children have the potential to be really dangerous; those are the ones that shoot people in school later on, and do crazy things with their anger.

I also think the vampire represents the animal part of ourselves, because we do have certain instincts and we very rarely use them. The few instincts that we still have to obey are eating, sleeping and sex, really. Otherwise we're like walking heads.

The vampire myth has also been a symbol for the sexual act, with the beast conquering the beauty, but I didn't want it to be sexualised in our story. That's also the beauty of it: the innocence of this story. It's romantic in a non-sexual way.

Why do you say that?

Oskar doesn't mind that Eli's not a girl, or a boy, or anything. She's just androgynous. In the book there is a flashback, where Eli is castrated, 200 years before, and it was a nightmare to figure out how to do that, visually.

 I didn't want to bring in people with wigs and 18th century clothes. I didn't even want to put in a flashback, because that's a big thing to do, storytelling-wise. But it's a very important matter that she, or he,  asks Oskar, 'Would you love me if I wasn't a girl?'

A lot of the violence is implied rather than seen. Was that always your intention?

I would say that the strongest images of violence are the ones you make yourself, in your head. If you were to try to film a scene with a small kid tearing someone's head or arm off, with a limited budget, it would be rather impossible, I think.

The film has been a big hit with horror audiences. Did that surprise you?

That was a big surprise to me. I have been travelling around, meeting these guys with long black hair and rings in their noses, introducing themselves as 'Bloody Hell'. (Laughs) I didn't know it was so big, this community!

It's also been a critical hit in the US, where vampire movies are suddenly in vogue. Have you seen Twilight yet?

I've only heard about it. (Pause) Suddenly, it seems everybody wants to tell a vampire story. It's really strange; we started this process four years ago, and nobody was talking about vampirism then. It's the mystery of synchronicity, I suppose. I really couldn't give you any other explanation!

Let The Right One In is in cinemas across the UK from 10 April