For the film, actress Josie Ho did not work with a script. Director Johnnie To wanted the actors to come to the set with their minds clean, so he could draw whatever he could on them. Ho did not find this method of improvisational acting difficult.
The hotel the characters return to numerous times, and where also the final showdown takes place, was in fact a completely roof-less set built on top of the Milkyway Image (the film's production company) building. Scenes taking place in the hotel were then filmed at night, resulting in an impressive ambient lighting.
Hong Kong's Official Submission to the Best Foreign Language Film Category of the 80th Annual Academy Awards (2008).
The red car the gang steals and uses bears the license plate "MF 97 99". The two numbers refer to the two dates when Hong Kong (where the gang comes from) and Macao (where the film takes place) became Special Administrative Regions of China: respectively July 1st, 1997 and December 20th, 1999.