A new film by David Cronenberg is always an event, and perhaps more so following the commercial and critical success of A History of Violence, his 2005 hit starring Viggo Mortensen as an apparently ordinary family man with a very dangerous past.

Since that film (which went on to two Academy Award nominations) had its debut at Cannes, Cronenberg's name has been linked to a number of other projects, including a sci-fi story called Painkillers, which he was to write.

He was also supposed to be involved with an adaptation of Martin Amis's novel London Fields and a Hollywood-set film, Maps to the Stars, written by Bruce Wagner (Wild Palms).

But instead of these, we have Eastern Promises, a London thriller involving Russian mobsters dealing in sex-slave traffic.

Mortensen returns as a smooth underling to a crime boss (Armin Mueller-Stahl) and his out-of-control son (Vincent Cassel), who is charged with making sure an innocent midwife (Naomi Watts) does not reveal secrets she has inadvertently learned about the ruthless crime family.

The 64-year-old Canadian director, dressed in a black jacket and T-shirt, with grey hair styled in a characteristic Samuel Beckett-like stand, is, famously, an amiable, calm man who makes movies that often show extreme portrayals of sexuality and violence.

With Eastern Promises, Cronenberg has delivered what appears to be a second almost-conventional thriller in a row.

Not only will it play the Toronto International Film Festival in September, but it will open two other major international film festivals, The Times BFI 51st London Film Festival and the 55th annual San Sebastian Film Festival in Spain.

As for the other projects, they've all gone away. He "fell out of love" with his own screenplay, Painkillers, and one way or the other, all the other projects have faded away. In some cases, he says, rumoured projects have been news to him.

"It's sort of an IMDB [Internet Movie Database] problem, you know. Things have a reality on there and it's very difficult to get them off."

Eastern Promises was written by Steven Knight (Dirty Pretty Things) and was developed by BBC Films, where it had been hung up in development for some time before Cronenberg received the script through an agent. Robert Lantos's Serendipity films also became involved, making this a British-American-Canadian production.

Cronenberg immediately suggested Mortensen for the role of the Russian gangster "because I always felt he had a very Russian face."

But first, he felt there had to be more work paring down Knight's script: "What it was, was a first draft - and he tried a whole bunch of different ideas in the same script and not all of them worked ..."

Neither British, nor Russian, Cronenberg may not have been the most obvious choice for the film, but he says an outsider has advantages: "You can look at it with a pretty cold eye, which I think is a good thing. There's a lot of political correctness in England, and I don't care about that. It rolls off my back. And, of course, there's the class thing, which is always present. London is very segmented and people have strong ideas about different areas, but this was not going to be a posh movie. This wasn't Notting Hill. We shot in a lot of areas like Harlesden and Hackney where films aren't usually shot - immigrant places, dangerous places, not glamorous places, but they're full of history and full of life."

Mortensen, who travelled to Russia to live for a while to prepare for the role, sent Cronenberg a book about Russian criminal tattoos, which inspired Cronenberg to work them into a subplot. For Cronenberg devotees, it will be a classic example of the kind of body transformation themes have characterized his work for more than 35 years. But the scene that will undoubtedly prove the major talking point of the film is a bathhouse fight between Mortensen and two other men that is loaded with extreme violence along with nudity.

Initially, Cronenberg and his set designer had found what they thought was a perfect London bathhouse with lots of small rooms and corridors that was "creepy and great," but when the owners decided to renovate and modernize it, the filmmakers had to build their own. Working initially with models, he told the stunt co-ordinator what he wanted. The stunt co-ordinator spent a couple of hours a week with the three actors involved, with Cronenberg dropping in to monitor the action. Then the actors performed it in slow motion while Cronenberg worked out his camera moves: "I didn't want Bourne [Ultimatum]-style impressionism where you don't actually see what's going on. People go to the movies to transform, or live another life, and I wanted them to feel that they were there, that they were vulnerable."

Somewhere in there, he says, there was the question of what to do with the towels.

"I'll fight naked. That's how it would happen," Cronenberg says of Mortensen's reaction. "There wasn't even a discussion about what else we do because Viggo knows I'd be too restricted if I couldn't shoot him from the waist down."

Cronenberg says he's already got feedback commenting on the bravery of the scene. "These days with DVDs and screen grabs and so on, we know there will be naked shots of him on the Internet, so the naturally vulnerability of the actor is increased, but we also know that's how the scene must be played."