"You say, 'Well, where's Viggo today?' " says David Cronenberg, recalling the conversation that happened more than once on the London shoot, last year, of the exceptionally fine new thriller, Eastern Promises. "And they say, 'Oh, he's in St. Petersburg.'

"And you say, 'What!? I thought he was at the hotel.' "

Viggo, of course, is Viggo Mortensen, and he was in St. Petersburg, Russia, watching and walking, sipping tea and soaking in the culture, on his days and weekends off. In Eastern Promises, you see, he's a guy named Nikolai, a driver for the Vory, a criminal fraternity of Russian ex-pats operating out of London. And Mortensen, who plays opposite Naomi Watts (an English midwife with a Russian heritage), Armin Mueller-Stahl (a Russian restaurateur and mob boss) and Vincent Cassel (a Russian thug), was intent on getting the accent, the body language and everything just right.

"He does it in such a casual way," says Cronenberg, who first worked with Mortensen on 2005's A History of Violence. "He doesn't involve everybody else in it, he doesn't make it a big deal. . . . And he went without any guides, without any translators, interpreters. He could've had whatever he wanted, but what he really wanted was to be unobserved. He wanted to go and be anonymous and see what Russians are like when they're just being Russians. And that's invaluable for him."

Eastern Promises, which opened Friday, after premiering last week at the Toronto International Film Festival, is in some ways typical Cronenberg: cool, bloody, a little bent and weird. But it's also different: It's the Canadian writer/director's first film that doesn't include a single scene shot in his hometown, Toronto (even Spider, set in England, logged five weeks of interior shooting in Canada), and it's his first collaboration with Steve Knight, the British screenwriter of Dirty Pretty Things.

"The irony is that this script was written before Dirty Pretty Things," Cronenberg says of the Stephen Frears-directed drama about Turks, Nigerians and other immigrants trying to get by in London. Eastern Promises, too, is about "embedded cultures within other cultures."

Set in an opulent Russian restaurant, and in London brothels where young Russian women from benighted, backwoods towns are held captive and forced to have sex, the culture of the old country permeates the film.

"They think about their homeland, they talk about it, they yearn for it, but we never see it," Cronenberg explains.

But Mortensen saw it. And he brought back a book that wrought significant changes on the Eastern Promises screenplay. It's called Russian Criminal Tattoos, a tome full of photographs of elaborate body art that Russian mobsters cover themselves with - each star-pattern, or pipe-smoking kitty's face symbolic of some sinister accomplishment.

"The tattoos had been alluded to in the script, but not really gotten into in any depth, and when I read [the book], I sent it to Steve Knight and I said, 'This is going to blow your mind . . . this really should be a central metaphor in the movie.' "

And so there's a scene in which Mortensen's Nikolai is inducted into the Vory, sitting back in a restaurant banquette, getting etched by tattoo needles. "It gave us all these fascinating details, and opened up the discussion [in the script] about renouncing your mother and your father and your country as well, because you now belong to us, the brotherhood, the Vory, the thieves-in-law."

Cronenberg shot in areas of London little seen in movies. He never set foot in Notting Hill. And he cast Watts, English-born but Australian-bred, without ever testing her opposite Mortensen. In fact, Cronenberg hadn't met the actress until she showed up to begin work.

"I just thought she was the perfect match for Viggo," he says. "You're like a boy scientist, because I knew that I wouldn't have a chance to ever see them in a room together before I saw them on the set. She was off making another film. . . . So, you have to imagine them together. I spoke to her on the phone, and we exchanged e-mails. . . . And based on talking to her and seeing her movies, I just had to intuit that she and Viggo would be a terrific match."

Which can be a risky thing. What if they'd met and there wasn't any chemistry, no there there?

"As a filmmaker you are literally making two or three thousand decisions a day," Cronenberg says. "No exaggeration. Everything from the fork and the spoon and the color of the clothes, the angles of the camera and the lens that you're going to use and the lighting, and your casting. . . . So, you're used to making decisions and you're used to living with the consequences of those decisions. I mean, some things you can change. [But] casting of a major actor you very rarely can change. . . .

"Even if you've seen them and thought they were great in some other role, you haven't seen them in this role. And you don't really know for sure. Sometimes you find - it's rare, but on occasion - you'll find that an actor will fight the role even though he or she was excited to play it. But for some reason, now, they're fighting against it. Many strange things can happen that are unpredictable, and you just have to have confidence in your intuition, because that's really all you've got. There's no guidebook for these things."

Cronenberg, 64, isn't sure what he'll be working on next. He has old scripts floating around at studios, at producers' offices. And his agent keeps a steady stream of potential projects coming his way.

But whatever he does next, he'll be happy to work with his History of Violence/Eastern Promises leading man.

"Well, I'd like Viggo to be in any and every movie that I do, frankly," he offers with a laugh. "And if I can possibly find a role for him in anything I do, I will. That's my promise. That's my Eastern promise."