Production notes

Writer Alan Mak had long wanted to write a story about police and gangsters. The script of Infernal Affairs was inspired by John Woo's Face/Off (1997) but Mak knew that its science fiction elements were too implausible in reality, so he focused on the exchange of identity and psychology between the two leads as its themes. Additionally, Mak wanted to explore the idea of triad members going undercover within the police, which had not been the plot of a movie prior to the release of Infernal Affairs. In the process of Mak's creation, his good friend Felix Chong also encouraged and supported him. The script, written by Mak and revised by Chong, took three years to complete. The dialogue in the famous rooftop showdown was created on the spot by Felix Chong and Tony Leung, with Chong playing Andy Lau's part. The script originally included a typical shootout in the third act, but Leung insisted on turning it into a dialogue scene. Gordon Lam did not receive the full script and did not know his character was also a triad mole until the final scene.

The script for Infernal Affairs originally belonged to Andy Lau's Teamwork Motion Pictures, but due to the company's entanglement in a lawsuit at the time, it was unable to produce the film. In addition, the creative team lacked investors because other studios viewed a gangster film as no longer novel or profitable. This hesitation reflected broader industry tensions. Professor Gina Marchetti noted that Hong Kong filmmakers like Lau experienced fierce competition from emerging Hollywood, while also battling widespread DVD and VCD piracy. Eventually, Andrew Lau made a hopeless bid and showed the script to John Chong at Media Asia Entertainment Group. To his surprise Chong and company chairman Peter Lam saw potential in the story. Lam proceeded to invest HK $20 million in the film, under the condition that Andy Lau will star the film. In total, the budget for the movie ended up being HK $50 million, with over half going into production costs to amplify the quality of the film. This allowed for the usage of a full orchestra for the score of the film, as opposed to a synthesizer, which marked the first time a full orchestra was used in Hong Kong cinema.

Upon its premiere in Hong Kong, Infernal Affairs grossed $160,356 during the opening day (16–19 January). However, the release in 2002 coincided with a decline in the Hong Kong film industry, with local film revenues dropping by 24% compared to its previous year. The following year marked the SARS outbreak, with only 77 films released. From 2005 to 2009, annual box office takings from local releases declined to around US$25 million and US$32 million. Scholar Yiu-wai Chu pointed out that during the downturn in Hong Kong cinema, piracy was rampant, and competition from imported Hollywood films was fierce. Against this backdrop, Infernal Affairs became the economic savior of the Hong Kong film industry, revitalizing confidence in Hong Kong's box office and proving that local films could still dominate the Hong kong market. In total, the film grossed $7,035,649 during its run in Hong Kong theatres. The film was then released across Asia, where it grossed a further $169,659 from theatre receipts. In 2016, South Korean theaters re-released the film, which went on to gross $128,026 across three weeks. The total lifetime gross of the film in Korea is $977,903. In total, worldwide, the film grossed $8,836,958 across release in both domestic markets and European theatres which displayed the film.

On Rotten Tomatoes, Infernal Affairs has an approval rating of 94% based on reviews from 64 critics, with an average rating of 7.50/10. The consensus from the site reads as "Smart and engrossing, this is one of Hong Kong's better cop thrillers." On Metacritic, the film has a score of 75 out of 100 based on reviews from 19 critics, indicating "generally favorable reviews". It was ranked as the 62nd Best Movie of 2004, 86th Most Discussed Movie of 2004, and the 95th Most Shared Movie of 2004. Film critic Roger Ebert gave the film a three-out-of-four star rating and described Infernal Affairs as offering "rare emotional depth." In his words, "The movie pays off in a kind of emotional complexity rarely seen in crime movies. I cannot reveal what happens but will urge you to consider the thoughts of two men who finally confront their own real identities—in the person of the other character." New York Times reviewer Elvis Mitchell was so enraptured with the film that he stated that "Infernal Affairs is so beautifully shot that the images occasionally distract you from the condensed policier plot." The film has also attracted comments from the academic community. Scholar Yiu-wai Chu believes that the unique feature of this film is that it reflects the psychological state of Hong Kong people after 1997. Through the identity crisis of two undercover agents, the film expresses the anxiety of people in Hong Kong after its historical transformation. Furthermore, the film breaks away from the formulaic approach of traditional Hong Kong gangster films that focus too much on action scenes, instead shifting its focus to the psychological level.

Infernal Affairs played an integral role in Andrew Lau's breakout films in entering the 21st century. Being the most critically acclaimed film of his to date, it was ranked No. 30 in Empire's "The 100 Best Films of World Cinema" in 2019. Infernal Affairs gained significant traction during its festival run as it was nominated for sixteen awards during the 22nd Hong Kong Film Awards, winning seven of those categories. It also won the Best Picture at the 40th Golden Horse Awards, the 8th Golden Bauhinia Awards, and the Best Foreign Language Film at the 46th Blue Ribbon Awards. Eventually, Infernal Affairs would spark the creation of two more films, with Infernal Affairs II getting 11 nominations and Infernal Affairs III getting 7 nominations during the 23rd Hong Kong Film Awards, with Infernal Affairs II winning Best Original Film Song.

With star power, visual allure, and an engaging script, Infernal Affairs did very well critically and financially, spawned two sequels and a television series, and attracted the attention of Hollywood. In 2003, Brad Pitt's production company Plan B Entertainment acquired the rights for a Hollywood remake, named The Departed, which was directed by Martin Scorsese, written by William Monahan, starred Leonardo DiCaprio, Matt Damon, Jack Nicholson, Mark Wahlberg, Martin Sheen, Ray Winstone, Vera Farmiga, and Alec Baldwin, set in Boston, Massachusetts, and roughly based on the life of famed Boston mobster James "Whitey" Bulger. The Departed was released on 6 October 2006 by Warner Bros. Pictures and won four Academy Awards, including Best Picture, Best Adapted Screenplay for Monahan, and Best Director for Scorsese. The Departed takes place in Boston, portraying the city as a “Crime City” shaped by the activities of the Irish mob scene. Unlike the moral ambiguity explored in Infernal Affairs, The Departed offers a distinctive resolution of a moral divide. Andrew Lau, the co-director of Infernal Affairs, who was interviewed by Hong Kong newspaper Apple Daily, said: "Of course I think the version I made is better, but the Hollywood version is pretty good too. [Scorsese] made the Hollywood version more attuned to American culture." Andy Lau, one of the main actors in Infernal Affairs, when asked how the movie compares to the original, said: "The Departed was too long and it felt as if Hollywood had combined all three Infernal Affairs movies together." Lau pointed out that the remake featured some of the "golden quotes" of the original but did have much more swearing. He ultimately rated The Departed 8/10 and said that the Hollywood remake is worth a view, though according to Lau's spokeswoman Alice Tam, he felt that the combination of the two female characters into one in The Departed was not as good as the original storyline. Scholars have identified Infernal Affairs as a significant Hong Kong crime film of the early twenty-first century. Its exploration of divided identities, moral ambiguity, and undercover operations contributed to its international appeal and facilitated adaptations across different cultural contexts, most notably Martin Scorsese’s The Departed (Choy, 2007, pp. 52; Li, 2018, pp. 325–326; Marchetti, 2007, pp. 3–4). Academic studies have also linked the film’s success to a broader revival of Hong Kong commercial cinema and the post-handover cultural identity (Law, 2006, pp. 398; Marchetti, 2007, pp. 168). Media Asia released a limited edition of eight-DVD set of the Infernal Affairs trilogy in an Ultimate Collectible Boxset (無間道終極珍藏DVD系列(8DVD套裝)) on 20 December 2004. Features included an online game and two Chinese novelisations of the film series by Lee Muk-Tung (李牧童), titled 無間道I+II小說 and 無間道III終極無間小說. In 2009, a Korean remake City of Damnation, which was directed by Kim Dong-won was released on 22 January 2009. In 2009, a Telugu remake Homam, which directed and acted by JD Chakravarthy along with Jagapathi Babu was released and became a notable movie. In 2012, Double Face (ダブルフェイス), a Japanese television remake starring Hidetoshi Nishijima was released by TBS and WOWOW. The production aired in two parts: "Police Impersonation" on WOWOW and "Undercover" on TBS. A TV series remake debuted in 2018 produced by Media Asia and former TVB producer Tommy Leung. The series, which is titled Infernal Affairs like the film, stars Gallen Lo, Damian Lau, Paul Chun, Lo Hoi-pang, Eric Tsang, Derek Kok, Dominic Lam, Toby Leung and Yuen Biao. The story takes place years after the films' events, with some minor characters reprising their roles alongside a new cast. The TV series uses the same concept as the film, but with an entirely new story and characters, and the setting expanded beyond Hong Kong to include Thailand and Shenzhen. It stretched through three seasons with each season consisting of 12 episodes. Hindi remake is going to be a joint development between Warner Bros. India and Mumbai – based banner Azure and is set for a remake for a two-picture deal

Lau, Tsang, and Jacky Cheung parodied the cinema scene to promote the Hong Kong Film Awards. Lau and Tsang, in their respective characters, go through the scene where they meet to gather info on the undercover cop amongst Hon Sam's gang. Lau Kin-ming asks Hon, "Why do we always meet in a cinema?", to which Hon answers, "It's quiet. No one comes to movies". Cheung comes out from the shadows behind them and says, "I don't know...quite a few people watch movies" and we see a slew of Hong Kong celebrities watching various clips of Hong Kong films on the screen. Originally Tony Leung was going to appear but scheduling conflicts led to the recasting. The 2003 TVB spoof celebrating the Chinese New Year called Mo Ba To (吐氣羊眉賀新春之無霸道), the 2004 comedy film Love Is a Many Stupid Thing by Wong Jing, and the 2004 TVB television drama Shades of Truth were re-writings based on the plot of the film. In Taiwan SHODA (劉裕銘) and a secondary school student Blanka (布蘭卡) cut and rearranged the original film and inserted new sound tracks to produce their videos Infernal Affairs CD pro2 and Infernal Affairs iPod on the web. The videos had many views and both producers removed their videos after receiving cease and desist letters from the Group Power Workshop Limited (群體工作室), the Taiwan distributor of the film. The hi-fi shop scene was later recreated with additions of excerpts of the film to encourage businesses to join the Quality Tourism Services Scheme in Hong Kong. The success of the film inspired many genres, including an open-world video game from United Front Games titled Sleeping Dogs (or True Crime: Hong Kong before being canceled by Activision Blizzard in 2011), with the protagonist of the story infiltrating the criminal underworld as an undercover cop.