Initially, Golden Princess did not want to make the movie, but Yun-Fat Chow (the studio's top star) insisted that it be made. Chow was also instrumental in bringing in Kong Chu. Chu had not performed in a movie since the 1970s and the studio was hesitant to use him, but he and Chow were good friends after having worked on some television shows together. Chow wanted Chu to play the part of Li, but Chu thought he was too old. Director John Woo suggested that they bring in his friend Danny Lee, who at one time wanted to be a cop and had already made a name for himself playing both police officers and gangsters. Chow had previously worked with Lee on a small film in the early 1980s called Zhi fa zhe (1981) (aka "Killers Two") and Ringo Lam's gangster classic City on Fire (1987), and agreed with Woo that he would be a perfect fit for the part of Li.
The scene where Jeff beats up Jennie's would-be attackers in the alley was tough for Yun-Fat Chow, who doesn't like violence. Director John Woo wanted hard hits, but Chow had trouble at first. After some coaching from Woo, Chow was able to muster up anger to make the scene more convincing. In fact, it became too convincing, as the stuntmen had to tell Chow to pull his punches a bit after one of them got hurt. Chow got hurt himself during the filming of the church shootout, when a piece of plaster cut his face, missing his eye by an inch. You can see the cut during the part where Jeff and Li talk before leaving the church.
All of the guns in the film are real. Because of Hong Kong's very strict gun laws, they had to be specially imported, and their use on-set was closely monitored. The gunfights in the streets of Hong Kong drew complaints from residents. Many local police officers are John Woo fans, and they usually let him keep filming. The shootout on the tram caused chaos in the Causeway Bay district; people thought a real robbery was going on. Woo had to talk to the Police Superintendent himself before he was allowed to resume filming.
John Woo improvised all of the action sequences on the set, with the actors, stuntmen and stunt director. He never used storyboards, partly to prevent his ideas from being stolen and partly because, according to the DVD commentary, he prefers to "work as an artist, like a painter. I want to show where my mood takes me." When Woo planned non-action scenes, he often changed things at the last minute. For instance, most of the dialogue in the apartment when Sydney double-crosses Jeff was improvised.
Hark Tsui was extremely unhappy with this film and wanted to have it completely recut. For example, he felt that the focus of the movie should be on the cop instead of the killer. Therefore, he wanted the film to start with the scene that introduces the cop. The shootout in the restaurant, during which the killer blinds Sally Yeh, was to be completely cut and only inserted in flashbacks later in the movie. Neither director John Woo nor editor David Wu were going to re-edit the film to Tsui's demands and, due to a tight schedule (the film was going to premiere in Taiwan in a short time and some 100 cinemas had already booked it), Hark didn't have the time to mess with the film. The movie was a huge success when it premiered in Taiwan, which made Hark so furious that he (allegedly) threw things out of his office window.
Because it was so hard to get permits, some of the footage for Tony Weng's assassination scene was shot under the pretense that director John Woo was doing a documentary about Hong Kong's annual Dragon Boat race. Woo shot the bulk of the footage five months earlier, and brought in a small crew later to fill in the gaps. Woo ended up editing the scene himself, which took three weeks. Woo is a huge fan of musicals, and tried to think of the sequence as a musical number or dance sequence. He even edited it in time to the soundtrack.
In the first versions with English subtitles, nearly everything in the film, including dialogue, the song and characters' names, was mistranslated. For example, Yun-Fat Chow's character is named Ah Jong, not Jeff or John. The first-ever accurate subtitled version released was the Hong Kong Legends UK DVD in 2002.
Jeff and Li's "Mexican standoff" in Jennie's apartment was inspired by Sam Peckinpah's The Wild Bunch (1969). John Woo notes on the Fox Lorber DVD commentary that Mad magazine's "Spy vs. Spy" also played a part in constructing this scene.
Hark Tsui and John Woo often disagreed about aspects of the film such as the opening scene, where Woo wanted the singer to perform a jazz song and have the killer playing a saxophone. Tsui rejected this idea, as he felt that the Hong Kong audiences didn't understand or like jazz that much. Woo stated, "I had to change it to a Chinese song, the kind of songs they always use in Hong Kong movies." The actress playing Jennie, Sally Yeh, is a popular singer but she also didn't find a slow ballad suitable for the film.
The statue in the police station (shown before Li's interrogation by his superiors after the tram shootout) is of Gen. Kwan, a soldier from over 1000 thousand years whose bravery and loyalty has made him like a god to cops and gangsters alike. The same statue was re-used for the CID Headquarters set in Hard Boiled (1992).
Miramax originally bought the rights to the film, as well as Hard Boiled (1992), and Bullet in the Head (1990) to be released on DVD, but it was scrapped because John Woo didn't want to approve Miramax's version to be released in a cut version in which could have a another uproar on the HK films being edited for a U.S Release.
During the assassination at the boat race, Ah Jong wraps a bandana around his left hand. At the time that this scene was filmed, Yun-Fat Chow was simultaneously working on A Better Tomorrow III: Love and Death in Saigon (1989); on the set of that film, he cut his finger on the front sight of an Uzi submachine gun during a stunt. Afterwards, he went straight to the set of this film to shoot the assassination scene, with his hand still injured and the bandana wrapped around it. John Woo decided to include this in the film.
James Horner's music from Red Heat (1988) is used throughout the film, as well as a cue from the training sequence of Hero and the Terror (1988). The director of "Red Heat" is Walter Hill, who was later going to remake this film before he pulled out.
John Woo approached his friend, Sally Yeh, asking her to be in a film to play an important female character. Yeh was currently contracted with Hark Tsui and accepted the role but later felt she did not give her best performance.
Scenes from the Dragon Boat festival were shot months apart, some footage was of the boat races and rest of the footage involving the actors was shot months later. It was planned for the boats to flip over during the chase but the owners refused because they felt it would bring bad luck.
The trivia items below may give away important plot points.
Jennie's ballads are in Sally Yeh's own voice. She was a popular pop star, and her concert schedule conflicted with the filming schedule. This led to a radical change in the storyline, with her blind at the beginning of the movie and telling the story through flashbacks, and a different ending, with Jennie flying to America for a cornea transplant.
A cat appears when Ah Jong first meets Jennie on her visit home, and secondarily when Li's partner Chang tries to catch Ah Jong in Jennie's apartment. In Chinese culture, a cat coming into a home is an omen of ruin and poverty for its inhabitants. Both Chang and Jennie meet negative outcomes in the film.
The original ending of the film involved Jennie waiting at an airport for Li to give her the money and for them to travel to the United States. Due to Sally Yeh's tight filming schedule, the scene was not filmed and replaced with Ah Jong playing the harmonica.