The scene in the bathroom, where Robert De Niro dunks his head in a sink full of ice, is in fact a real trick that real police detectives use to get over hangovers. De Niro discovered the trick during his research for the role, and was eventually written in.
The language spoken by Charlize Theron's character is Afrikaans, one of the official languages of South Africa. She says, "Hello Mr. Botha, can I call you back? You would never believe who just walked in here, it's Eddie Flemming. Thank you, good-bye." Botha was a famous boxer in the 90s from South Africa. Charlize was born in South Africa, and speaks fluent Afrikaans.
Edward Burns gained twenty pounds, to accurately play an Arson Investigator, as they have to have spent at least seven years in the fire service, and would gain muscle mass across their backs, chests, and arms.
Graham Knuttel, the Dublin-born artist, was paid 50,000 pounds for a canvas which was destroyed in the film. The producers wanted the movie to be as realistic as possible, and so ordered the artwork to be destroyed in one scene.
Mindy Marin, who's the film's Casting Director, was featured in the part of Kim Cattrall's assistant, because she looked the part, and they weren't able to find an actress who was capable of playing the role prior to shooting, according to director John Herzfeld.
There are a lot of location inconsistencies throughout the film. Like the scene involving the apprehension of the Jamaican killer that Robert De Niro and Avery Brooks are going after early in the film, was clearly Los Angeles, and most of the scenes in Times Square, were shot in New York City, for example. Most of the interiors were shot on sound stages and actual New York City and Los Angeles locations.
While the film was delayed, J. Peter Robinson was brought on to provide a new beefed up original score for the film as the studio was not happy with what Anthony Marinelli had done musically for the film at that point. Marinelli had written about seventy minutes of music for the film. Robinson composed approximately thirty minutes of new material, and retained about half of Marinelli's contribution, which is why they share composing credit in the final credits for the film.
The tension between Emil and Oleg is based on the real tension between some Czechs and Russians. Many Czechs hate the Russian language, as they were forced to learn Russian under Soviet occupation. Karel Roden refused to speak Russian to Oleg Taktarov during rehearsals.
The film was originally slated to be released in the spring of 2000 by New Line Cinema, with theatrical trailers appearing in fall and winter 1999. For reasons unknown, the film was pulled from the spring 2000 schedule, and then delayed. The film was finally released in March of 2001. A similar situation would happen with Knockaround Guys (2001), which was slated to be released that spring, only to be yanked and delayed. That film would finally be released in late 2002.