A brutally violent scene in David Cronenberg's Eastern Promises has become the cause celebre of cinema in 2007.

In it, Viggo Mortensen is totally naked -- literally balls out -- while fighting two clothed killers in a bathhouse. The effect on audiences, obvious during the Toronto filmfest when the thriller made its world premiere last week, is shattering.

"The way it is executed is just unbelievable," screenwriter Steven Knight says with admiration of the scene he wrote as a barebones scenario.

"Even I looked away!"

Cronenberg turned his the scene into art, Knight says.

"It's David! The camera doesn't look away at the crucial moment. You see all of it, which is amazing, absolutely amazing.

"In my opinion, it makes violence extremely unattractive, which is great, that's the point."

Cronenberg concurs. "For me," the Toronto filmmaker says, "violence is the destruction of a human body. I don't want to let my audience off the hook. I want them to feel that."

Executing the scene took weeks, he says. "It's a long accumulation of details." The bathhouse was built in a studio; the scene was choreographed with the actors, not stunt men, to make it real and not ritualistic; the lighting and camerawork had to be harsh and realistic; the actors had to train in specific fighting styles chosen for their characters; and it took two days to shoot on location in London, England.

Above all, Mortensen had to be naked.

"I mean," Cronenberg tells Sun Media, "you can imagine how silly it would be if he had this towel around him in a fight scene in the sauna. He has enough of a director's sensibilities that he understood how restricted I would be if I'm only going to shoot him from the waist up or if I'm going to have to worry about his balls every second.

"I want to see everything! I want the audience to feel that it's happening to them, which is scary. It's knives and you're naked!"

Mortensen says that, contrary to popular belief, Cronenberg is extremely responsible about violence.

"The reality is that he takes very seriously the depiction of any violence. That's why he shows it honestly and straight-forwardly.

"He just shows you there is damage and it's brutal and there are consequences, physically and emotionally, to any act of violence."

But there may be personal consequences for Mortensen.

"We're not naive," Cronenberg says.

"We all know the Internet. We all know that people could do screen grabs from DVDs and that an actor has a certain vulnerability now that actors never would have had before.

"Viggo knows that. I know that. So he had to shrug that off and just say: 'Well, this is the way it had to be done!' "