After Raizo (Rain) is captured by Interpol and chained to the wall, one of the agents looks at him and says, "He looks like he should be in a boy band." Rain started his career in the boy band Fanclub.
Korean pop star Rain had no martial-arts experience prior to filming. He trained extensively for six months in martial arts and weapons handling with stunt and action choreographers Chad Stahelski, Peng Zhang, Jon Valera and Jonathan Eusebio as well as weights and cardiovascular workouts with personal trainers; he also went on a diet of chicken breasts and vegetables.
One hundred pounds of gold is 1458.333 troy oz. The bank transfer corresponding to the assassination of the Russian Prime Minister Zukoff was $1,555,999.90 - which meant the assassination occurred on a day when the $/Gold rate was $1066.97/oz - which was last on 4 Nov 2009.
The trivia item below may give away important plot points.
The condition that the Tattoo Master and Mika are seen to have (their hearts are placed on the opposite side of their chests) is in fact an actual existing medical condition known as "Dextrocardia situs inversus": the organs of the chest and abdomen are arranged in a perfect mirror image reversal of the normal position. Most people with dextrocardia have no medical symptoms or complications resulting from the condition and it only occurs in less than 1 in 10,000. NBA basketball player Randy Foye and actress Catherine O'Hara are known to have this condition.