Nawazuddin Siddiqui got a major breakthrough with this film as he struggled for 12 years in just small character roles. After Kahaani he became one of the busiest actors.
Popular Hindi actors were not willing to work with him Sujoy Ghosh after his two flops. He also believed that audience might expect more screen-time from a better-known actor. Hence he cast more Bengali actors in the film who were lesser known in Hindi films.
Sujoy Ghosh built Khan as a character with lean physical build but full of mental strength, loyalty and patriotism. Khan smokes a relatively cheap brand of cigarette (Gold Flake) despite his high official post; Nawazuddin Siddiqui had smoked that brand of cigarette throughout his struggling days.
The film was shot totally in Kolkata and most on the streets. Bengali actor Prosenjit Chatterjee had encouraged Sujoy Ghosh to shoot the film in Kolkata.