When the project was first announced, Vin Diesel was going to star as Agent 47. Despite Timothy Olyphant taking over, Diesel is still credited as executive producer.
The car Agent 47 drives is an Audi S5.
When Agent 47 jumps through the hotel window into the kid's room, they're playing _Hitman: Blood Money (2005) (VG)_ for the PlayStation 2. (They're playing through "Death of a Showman", the training level.)
Diana and Agent Smith, characters from the video games, make appearances in the film.
The movie was delayed from its October 2007 release, to late November 2007 to facilitate for re-shoots.
Timothy Olyphant underwent muscle and martial-arts training in preparation for his role.
The character of Nika (Olga Kurylenko) draws inspiration from a recurring female character from the games known as Lei Ling (changed to 'Mei Ling' in 'Contracts'). Lei/Mei Ling was a prostitute that 47 saved on two occasions, receiving vital information in return.
Olga Kurylenko was distraught after having to cut her hair for the role. She said she didn't feel like herself anymore, which she used to help her connect with her character.
When Agent 47 and Nika walk through the train station in St. Petersburg, to when Agent 47 is followed underground, the time in which the scenes were filmed switches between March 2007 and November 2007.
Director Xavier Gens was originally asked by the studio to make the movie PG-13, which he refused to do because of the source material. He was allowed to shoot the movie aiming for an R, however in the final stages, he was removed and Nicolas De Toth was brought in to re-edit the movie for commercial purposes, as he did with Live Free or Die Hard (2007) (he is thanked in the credits for this editing).
The production had originally tried to use Jesper Kyd's music, but instead opted for an original score by Geoff Zanelli.
During the restaurant sequence, the scene in which Nika (Olga Kurylenko) talks about owning a squirrel/chipmunk when she was young was actually true for the actress, and the scene was improvised during filming.
The scene where Agent 47 sits atop the roof, sniper in hand and red lettering behind him is taken directly from artwork from the game, as is the scene between 47 and Yuri in the bathtub, including the rubber duck.