The tattoos around Nikolai's - Viggo Mortensen's character - 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.'
One day after shooting, Viggo Mortensen went to a pub without washing off his tattoos or even changing out of his costume. He overheard a couple speaking Russian and decided to eavesdrop, to see if he could understand what they were saying. He caught a few words before they stopped mid-sentence. When he turned around, he saw they were staring at the tattoos on his hands; they were scared, as they assumed he was a real member of Vory v Zakone.
The scene where Semyon demonstrates his musical skills to the little girls was not dubbed. Armin Mueller-Stahl does play the violin in real life, and was a noted concert violinist in his youth.
Viggo Mortensen says his performance was informed by Vladimir Putin. He watched Russian TV daily to immerse himself in the language, and Putin was on the news constantly. Viggo noted his "unfazed look no matter what was happening, or that cat-that-swallowed-the-canary" expressions, his stance, and how he held his hands. He also took into account Putin's military and espionage background.
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.