Mira Sorvino speaks fluent Mandarin Chinese. Yun-Fat Chow is a Cantonese Chinese native speaker but can also speak some Mandarin. Sorvino was able to help translate for Chow who was just learning English at the time.
During production, Columbia Pictures felt that Antoine Fuqua was struggling to deliver suitable material and ordered a studio exec to be present during most of the filming to ensure that their money was being well spent. This angered Fuqua and made things tense between him and Columbia. 'Debra Hill' (II), a veteran producer, was called in by Columbia to cool things down. Lead actor Yun-Fat Chow stood by Fuqua the whole time and told the producers to trust him and his vision. The troubles didn't end after the production wrapped. When Fuqua delivered his initial cut, Columbia began testing the film. Test audiences struggled with the notion of a less than pure hero and the bi-racial relationship between Yun-Fat Chow and Mira Sorvino. They also had issues with most of the other characters back stories, so Columbia called in action editor 'Richard Francis Bruce' to tighten up the film. All romantic elements between Yun-Fat and Sorvino were removed, along with most of the characters' motivations.
Also the first American film for Til Schweiger who already was a major movie star in his native Germany at the time. Number of words his character, one of the titular replacement killers, speaks: Zero.
Fuqua and Chow decided to cut some of the dialogue originally written for his role - partly to focus more on the visual aspects of the film and partly because Chow's English wasn't that good back then.
In the music that plays over the introduction of the "replacement killers" as they walk through the airport terminal, the female voice that is heard singing is actress Claire Trevor singing "Moanin' Low" from the film Key Largo (1948).