To prepare for his role, Viggo Mortensen traveled alone to Moscow, St. Petersburg and the Ural Mountain region of Siberia, where he spent five days driving around without a translator. He read books on the gangs of the vory v zakone (thieves in law), Russian prison culture and the importance of prison tattoos as criminal résumés, and perfected his character's Siberian accent and learned lines in Russian, Ukrainian and English. During filming, he used worry beads made in prison from melted-down plastic cigarette lighters and decorated his trailer with copies of Russian icons.
For the bathhouse fight scene, the scene was choreographed with the actors (not stuntmen), the actors had to train in specific fighting styles chosen for their characters and it took two days to shoot on location in London. According to the DVD commentary, both Mortensen and Cronenberg agreed that Nikolai had to fight his would-be killers completely nude.
One day after shooting, Viggo Mortensen went to a pub without washing off his tattoos or even changing out of his costume. He claims that some of the patrons became very frightened of him, assuming he was a real member of Vory v Zakone.
The tattoos around Nikolai's ('Viggo Mortensen') ankles read "Where are you going?" and "What the fuck do you care?" in Russian. Mortensen thought that they were hilarious, that 'one foot doesn't respect the other'.
None of the characters who were members of the Vory v Zakone used a gun throughout the movie. The reason for this is that when doing research on Russian organized crime, David Cronenberg discovered that members of the Vory v Zakone typically prefer to use knives instead of guns. The rationale for this is that if Vory v Zakone members were arrested by police and questioned as to why they were in possession of such weapons, the suspects could evade suspicion by claiming that the knives were simply for linoleum cutting.
The nine actors in the Fellowship in Lord of the Rings all got tattoos that said "the nine" in elvish. Even though they promised not to show them on camera, Viggo Mortensen's elvish tattoo is visible on his left shoulder in the scenes where he has no shirt