IF you happen to be the downstairs neighbour of director Gareth Evans, right, prepare to have a mystery solved.

All those thumps and bumps on the ceiling were not, in fact, baby elephants attempting cartwheels, but rehearsals for The Raid, a new martial arts movie that's punching its way towards cult status even before its release next week.

"We've got a big office space and we have one section where we put crash mats down," explains Evans. "The poor people below us can just here thud, thud, thud all the time."

Another mystery is how someone with a name and accent that are as Welsh as the village of Llanfairpwllgwyngyllgogerychwy-rndrobwllllantysiliogogogoch comes to be in Glasgow, speaking in Indonesian, about a martial arts movie.

The first is simple. It's the Glasgow Film Festival, where The Raid, such was the internet buzz, was one of the first screenings to sell out. Second, Evans is married to an Indonesian and lives in Jakarta. Third, the 31-year-old has been a martial arts fan since he first saw Bruce Lee in Enter the Dragon.

The Raid is the story of a rookie police officer who takes part in the storming of a block of flats in Jakarta where the residents are the baddest boys in town. The presence of riot shields and AK47s apart, most of the fighting is done via the Southeast Asian martial art of Silat.

Iko Uwais plays the rookie who is handy with his fists and feet. I ask him, with Evans translating, to explain how Silat differs from other martial arts. All martial arts share certain traits, he says, with only so many ways to block and punch and kick. Silat stands out because of its speed and fluidity. A fighter can switch their approach in a flash, going from eye to eye fighting to floor level moves. In Silat, fights are more like dances.

Evans met Uwais when he was making a documentary about Silat. Uwais was a pupil of one of five masters Evans was studying, but it soon became clear this particular pupil stood out. "We noticed that every time he was on screen you wanted to watch what he did," says Evans. "There was a screen presence there. You could see the kids idolising him."

At the time, Uwais, 29, was a delivery driver. After Evans and his wife, who runs their film company, asked him to be in their new movie, Merantau, a martial arts star was born.

It was while trying to raise the money for the film after Merantau that the idea for The Raid came to Evans. A fan of "self-contained" thrillers Die Hard and Assault on Precinct 13, he wanted to make something set in one building. That also took care of the bane of filmmakers' lives in that part of the world – the rain.

As hinted at by its 18 certificate, violence is to The Raid what sea water was to James Cameron's Titanic. There's a lot of it about. A high injury level? "When you do a film like this obviously people are going to get injured," says Evans. "You can't expect it to be a clean sheet." But was it a Wales v England rugby match brutal or proper war?

"Somewhere in between," he laughs. They were "super cautious" about the stunts, but every now and then something went wrong. "It can be the most basic shot in the world but sometimes people can get injured. Uwais only had to turn and punch someone but his foot stuck on the floor and he twisted his kneecap." The worst injury was someone falling on to concrete. "He was okay in the end. Just a little bit of a headache."

Evans and his wife met when they were students. They had a long distance relationship after she returned home, got married, and she came to live with him in Swansea. "We weren't really settled. We loved Swansea, we loved our house, but just the Monday-Friday was the kicker."

He was doing video work for a Welsh language website and travelling a lot. His wife knew what he really wanted to do and got him a documentary gig in Indonesia. Decision time. "That was the turning point, because I thought okay, I've got to quit my job to do this, we've got to be out in Indonesia for six months, let's just do it and see what happens."

The steepest learning curve wasn't learning the language but going from low- budget documentaries with a handful of crew to bigger productions, such as The Raid. In a rare move for a director, Evans wanted his set to be as egalitarian as possible.

Above all he didn't want to seem detached. "I felt self-conscious of the fact I'd be the white guy coming in and making a film. I didn't want there to be any awkward feeling. And I wanted the crew to be able to come to me and tell me anything that was wrong."

Such has been the reception so far, The Raid has already been signed up for a US remake. Evans isn't entirely sure why it has caught on so much with audiences, but thinks it might be something to do with its style, which hails from the 1980s and early 90s martial arts movies.

"There's a certain old-fashioned quality to it. The editing isn't flashy, we keep it quite straightforward and just present the action as it is."

Then there's Silat, a martial art that somehow manages to look supremely elegant which being ferociously violent.

Evans will be an executive producer on the US remake and Uwais and Yayan Ruhian will be choreographing the fights again. No-one, as yet, has been cast in Uwais's screen role. Uwais thought Hugh Jackman a possibility, but the character is meant to be fresh out of the police academy and that might be a stretch for the 43-year-old Aussie. It doesn't need to be a fighter, says Evans. "Someone with a dance background too could do it. You can be a really good martial artist but a terrible screen fighter. When we did auditions for The Raid and Merantau we'd see a lot of people who would be brilliant martial artists, super-skilled, know everything inside out. But when you put a camera on them and ask them to fake a fight scene, like a kick to the face or a reaction, sometimes they can't do it. There's performance to it, it's not just the technique."

What does he feel about some other director taking over his baby? "It's flattering as hell. To have a studio love your film enough to say they want to pump that much money into doing another version of it?"

The way he sees it, it's a win-win-win. The original will always exist and the remake will increase interest in it; Uwais and Ruhian can bring whatever they learn in the US home to Indonesia; and finally, the money will come in handy to make the next film, a sequel to The Raid.

"After spending two years trying to get one project off the ground it's all good," says Evans.

The Raid opens in cinemas on May 18.