If you pay attention through the movie, you can tell when or not Cheng will kill in a fight. When it's just a "fun" fight, he would wear either brown or blue pants with a t-shirt and a blue sash. But in a serious fight, he'd wear a long sleeve shirt with black pants and a white sash. This may be because in Chinese culture, white is a symbol of death.
The Thai brothel featured in the film was actually a genuine and functioning brothel. The extras who feature in these scenes (excluding Malalene's character) were actual prostitutes who were paid more by Golden Harvest than they would normally receive in a day by their clients so that they could appear in the film.
Bruce Lee was originally against two of director Wei Lo's ideas used in the film. First, was when one of the foremen was to be punched through a wooden wall. Wei wanted to leave the villain's outline in the wall, similar to something in a cartoon. Lee tried his best to change it, but somehow Wei got the upper hand. The second, and the most famous scene of the film, is the climactic "jump kick joust" between Lee and villain 'Yin-Chieh Han'. Lee, once again, didn't like the idea due to its separation from realism. However, he gave in, and the shot was done.
While not mentioned in the film, a few books released in the '70s at the time of the film's release mentioned that Cheng Chao An was forced to make his promise to his mother after his father was killed in a fight. In order to continue the family name, Cheng's mother wanted to make sure he would not fall to the same fate and that he'd live to raise his own family. Film historian Bey Logan even mentions this in an audio commentary.
The film was based on the true story of Cheng Chiu-on who fought the tyrants in Thailand. Cheng lived at the end of the 1800s and beginning of the 20th century. A memorial statue of him was erected in a garden in the Bangkok more than 80 years ago.
The mansion of the film's villain was, and still is, a Thai mausoleum. Usually, due to Chinese superstition, another location would be chosen to avoid a movie from being cursed. However, whether it was time restraints or the appearance of the location, it was not made public that it was a mausoleum until the film was completed.
The infamous saw scene (mentioned in the alternate versions section) apparently was shown only once to an open public and that was the film's original premiere in the fall of 1971 in Hong Kong. Co-star Maria Yi herself said it was present in the film but didn't look very realistic. Ironically, this was part of the rumor for years as to why the scene was never shown again.
This is the only Bruce Lee film (excluding his childhood films and Green Hornet re-edit films) missing his use of the nunchucks. They were not used by Lee until The Chinese Connection (1972). The only Lee trademark present in this film is his triple kick attack (Lee Sam Geuk - Three Legs Lee). Also, this is the only Lee film to be censored in its original country due to graphic violence.
Despite being credited for the music score in every release of the film, Wang Fu-Ling actually only composed music for the original Mandarin version. Peter Thomas composed the score for the English dubbed versions while Joseph Koo composed new tracks and chose stock music (including music from Don Peake's score for the original The Hills Have Eyes) for the Cantonese dubbed version in the early 80s.
The original director was Ng Gar Seung, however, he was replaced by Lo Wei a few weeks into production. The original star was also James Tien, who plays Hsiu Chien, while Bruce Lee was to be a co-star. However, when directors changed, the stars switched, giving Lee top billing. This may also be part of the reason why Lee does not fight until halfway through the film.
The international (English) title of this film was "The Big Boss". In the United States the English dubbed version was originally to be released under the title "The Chinese Connection", a play on the title of the highly popular film The French Connection (1971). For some reason the title was changed to "Fists of Fury". As a result, to avoid confusion with Bruce Lee's following film The Chinese Connection (1972) (known elsewhere in the world as "Fist of Fury"), the latter film's title for its U.S. release became first "The Iron Hand" and then "The Chinese Connection".
There have been at least three publicity photos in which Bruce Lee is shown attacking stuntman Peter Chan (he is shown as the man wearing a tucked in blue t-shirt and blue jeans) with a flying side kick during the fight with The Boss's remaining henchmen in the finale. However, Peter Chan's character was not present in this fight whatsoever in the final film. One of these photos is most widely seen on the back of the original CBS-FOX VHS release of the film... and is also horizontally reversed.