Using the shotgun in the rose box was an original idea in both this film and Terminator 2: Judgment Day (1991). It is a coincidence that they both came up with it at the same time. Its appearance in this film is not a reference to or a copy of "Terminator 2". It was used in two influential earlier films: Dog Day Afternoon (1975) when Al Pacino's character brings a rifle into the bank, and before that in Stanley Kubrick's film noir classic The Killing (1956), when the gang smuggles their heist gun into the track locker room hidden in a box of roses.
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, 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.
More than 200 guns were used in the film, all of which were real. Due to Hong Kong's strict gun laws, the weapons had to be imported specially from England and inspected by the HK police before they could be used on set. The production also had to import a substantial quantity of blank ammunition; in total, over 100,000 rounds of blank ammunition would be fired during the filming of the movie.
During the "hallway shot out" between Tony and Mad Dog, Tony Leung (Tony) was cut in the eye by a shard of glass. It happens when the two men are shooting each other from opposite sides of a set of glass windows. If you pay attention, Leung suddenly covers his face with his arm, an indication of when he was hit with the glass. Leung was given a day off to get medical attention and returned to the set to finish filming without problems.
_Hard Boiled (1992)_ was John Woo's last Hong Kong film before going to Hollywood. According to the Dragon Dynasty commentary, it was never meant to be Woo's last film there. It was somewhat successful in Hong Kong but not as successful as Woo's earlier films and it was at this point that the Heroic Bloodshed genre which Woo and a couple of directors were famous for was coming to an end as the Hong Kong audience were more interested in comedies, etc. However, when John had heard that this movie had extreme positive reactions from the American audience (which was rare at the time), that was when he made the decision to go to Hollywood. From there, he made his first American film Hard Target (1993) and continued with Broken Arrow (1996), Face/Off (1997), Mission: Impossible II (2000), Windtalkers (2002), and Paycheck (2003). In 2007, he went back to China to direct his epic period film _Red Cliff (2008)_ and produce and co-direct _Reign of Assassins (2010)_.
The teahouse where the first sequence was filmed was demolished five days after Woo was done. During filming, the neighbors called the police every night to complain about the gunfire, but the cops were fans of Woo, so they allowed him to complete shooting every night.
Like many of Woo's other films, elements of the film's action sequences were improvised, such as the famous "staircase slide" in the teahouse. Yun-Fat Chow came up with the idea of using flour in the climax of the teahouse shootout.
The scene at the jazz bar after the teahouse shootout was added on the last day of shooting. Chow wanted to have a scene that showed his real-life friendship with Woo. The scene was scripted and shot in less than a hour.
Originally, Philip Kwok (Mad Dog) was not supposed to have an acting role in the film, instead just working as the action director. But Woo was a fan of Kwok's acting work from the 70's and he felt Johnny Wong would not project enough of a real threatening presence, so he wrote in the character of Mad Dog and offered Kwok the part.
The statue that Tequila and the other police officers worshipped in the police station is of Guan Yu (aka: General Kwan, Taoist God of War). Guan Yu was a great military general, later an Emperor, during the Eastern Han Dynasty who was deified as early as the Sui Dynasty. Even today, General Kwan (in Cantonese, he is referred to as Kwan Gong) is still worshipped among Chinese people. Because Guan Yu is a guardian deity that exemplifies bravery, righteousness, leadership, protection and triumph, he is commonly worshipped by Chinese law enforcement agencies so that Guan Yu's blessings will protect them in life and in work. In Hong Kong, every police station has a shrine dedicated to Guan Yu (though worship of Guan Yu is not enforced). In the film's subtitle track, Guan Yu/Kwan Gong is incorrectly referred to as "Guana".
The production stopped for a month after the filming of the teahouse sequence due to script problems. Woo was having problems with Tony Chiu Wai Leung's character; he wanted to show how far he felt HK society had fallen and thus originally Leung's character was to be a psychotic villain based on a real-life psycho who poisoned baby formula. However, (even though Leung was willing to do the role) Yun-Fat Chow felt the role would be bad for Leung's career, and urged Woo to change the script. Woo agreed, and had his friend Barry Wong do a rewrite. Wong died before he could finish the script, so Woo had to complete the script during filming.
Most of the script was radically rewritten a week before shooting began. The original plot involved a man poisoning bottles of baby formula (hence the hospital locales in the entire second half), but director John Woo found the idea repugnant and decided to jettison it.
The "hallway shot" in the hospital was put in to break the monotony of filming. It was set up in about a day and a half. The crew had to all sit in one little room and watch the action via a series of monitors strategically placed along the hallway. Steadicams had proven to be too heavy, so Woo had his cameramen use hand-held cameras. Woo wanted to do the shot all in one take, but money was running low, so (even though the crew offered to work for free) he had to splice two shots together.
According to the Dragon Dynasty DVD Commentary, Michelle Yeoh was originally slated to play opposite Chow-Yun Fat as one of the heroes, with Tony Leung as the villain (when the plot hinged on a madman poisoning babies). However, scheduling conflicts prevented Yeoh from taking the part and the scene was re-written as Tequila's girlfriend with Teresa Mo in the part.
According to the commentator on the Dragon Dynasty DVD, many of Johnny Wong's underlings in the hospital shoot out were stunt men who had all worked on kung-fu movies for the famous Shaw Brothers studio.
The trivia item below may give away important plot points.
According to Dragon Dynasty's interview with John Woo, Tony Leung's character was originally slated to die from the gunshot wound to his stomach shortly after Tequila kills Johnny Wong (this perhaps was the initial intent of the spinning shot away from Alan's body on the street in the current film, which some have misconstrued to symbolize his soul departing). But the filming crew involved in shooting the scene was emotionally sad with the outcome and pleaded for Woo to let Alan live. As a result, Woo relented and allowed Alan to sail off into the Antarctic.