Cast the Expert: Philip Chan, who played Superintendant Pang, was a CID cop in the '70s and for a while was the head of one of the first undercover units of the Hong Kong Police.
Creator Backlash: Anthony Wong wasn't happy with the film as he thought that his character, Johnny Wong, was a two-dimensional and thinly characterized villain. He also considered John Woo hard to work with.
The Danza: Anthony Wong plays Johnny Wong, the film's villain.
Deleted Scene: Around a half-hour of scenes were cut during filming, among them were a relationship sub-plot between Teresa and Tequila, of which there's a few bits left over in the final film, and Tequila playing the clarinet over Benny's grave. Due to these cuts, Chow Yun-fat felt his character was not as deep as Tony Leung's, and convinced Woo to play a mentor character to Tequila.
Fatal Method Acting: Averted. During the filming of the scene in which Tequila is running down the exploding hallway with the baby, John Woo wasn't satisfied that the explosions were big and frightening enough in the scene. According to Woo in his Dragon Dynasty DVD interview, he asked the special effects technicians to reset the explosives and give him the trigger. When Chow Yun-fat ran down the hall, Woo immediately set the explosives off, nearly incinerating Chow, who barely made it. According to Woo, Yun-Fat Chow exclaimed to the producer afterwards "John's trying to kill me! John's trying to kill me!". When Woo heard Chow screaming, he went up to apologize to Chow and saw that the back of his head and coat were in fact singed from the explosions.
Like many of John Woo's other films, elements of the film's action sequences were improvised, such as the famous "staircase slide" in the teahouse. Chow Yun-fat came up with the idea of using flour in the climax of the teahouse shootout.
Originally, Tony was not supposed to cry after killing Mr. Hui, but Tony Leung felt the character should show some sign of breaking down. Woo filmed the scene both ways and decided to keep the "crying take" in.
John Woo intended for the film to be less stylized and more gritter than his past works, citing the Dirty Harry films as inspiration. Michelle Yeoh was to play Tequila's partner on the force, and Tony Leung was picked to play the villain, a psychopath poisoning baby formula. Woo was talked out of the idea, and a new plot was written with Leung playing an undercover cop.
A number of story elements were improvised during filming and not part of the original script, such as the character of Mad Dog, who came about as Woo felt that Johnny Wong didn't pose enough of a physical threat to the other characters.
The ending originally had Tony dying from his self-inflicted Shoot the Hostage gambit, but during filming the cast and crew begged Woo not to end the film on such a bitter and tragic note, and convinced Woo to rewrite the ending.