Tony Leung Chiu-Wai
The idea led to a script, but when the script was finally completed, Mak felt uncomfortable with the concept of another male buddy film. Chong and Mak decided that the film would center on a female police officer, and immediately thought of Sammi Cheng being cast in the lead role. The directors both felt that Cheng was a role model among women. Chong had thought about casting Eason Chan, while attending one of his concerts in 2006. During filming, the two directors taught Chan to use a method of acting similar to that of Tony Leung Chiu-Wai. Chong and Mak also revealed that the suit Cheng wears in the film was custom-made and modelled after the suit Andy Lau wears in "Infernal Affairs".