- When Edison was filming "Infernal Affairs", due to his poor work attitude, he did not seem to be awake during the filming. As a result, Liu Weiqiang was slapped.
- "Infernal Affairs" is the first collaboration between Andy Lau and Tony Leung since filming the TV series "Five Tigers Breaking" in 1991.
- Because the design of the protagonist of "Infernal Affairs" is very complicated, the design of the personality of the other supporting characters in the film is not too complicated.
- Andy Lau and Tony Leung in "Infernal Affairs" are very interesting. Andy Lau is a gang member, while Tony Leung with a moustache is a policeman.
- During the filming process, Tony Leung, who wore black leather jackets, black pants, and beard, had no action points, but he still had to touch up makeup from time to time in hot weather.
- Once Xiao Yaxuan and Tony Leung acted in a rival drama. Before the director officially called to start filming, Tony Leung was already in the play. He stared at Xiao Yaxuan intently, but Xiao Yaxuan did not dare to face his eyes. After the filming officially started, when Xiao Yaxuan and Tony Leung looked at each other, she was so nervous that she almost fainted.
- "Infernal Affairs" hired Shawn Yue and Edison Chen to play the roles of Chen Yongren and Liu Jianming, respectively. During the shooting, Edison Chen kept repeating actions, causing damage to the left clavicle due to friction with the spear, and Shawn Yue was trapped in the secret room playing sandbags.
- At the beginning of the filming, the Hong Kong police felt that the subject matter of the film was sensitive and feared that it would damage the image of the police force, so they wrote to the director to make slight changes.
Infernal Affairs behind the scenes gags
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Chan Wing Yan: Should I salute you?
Lau Kin Ming: No, don't. How long have you been an undercover?
Chan Wing Yan: I've followed Sam for 3 years; I had several other bosses before. All together, it's been 10 years
Lau Kin Ming: 10 years? I should salute you, instead.
Chan Wing Yan: I just want an identity. I want to be a normal man.
Lau Kin Ming: Getting tired?
Chan Wing Yan: You've never been a mole. You won't understand. Too bad I still can't find the stooge. I'll take him down if I've found him.
Lau Kin Ming: Don't worry. Let me give you back your identity, I'll open your file, but I don't have the password.
Chan Wing Yan: What's the Morse Code for undercover?
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Mary: I can't finish the novel, I don't know whether he's good or bad.