The lead character's name, Hoshino, was the nickname Don Wilson competed under exclusively when kickboxing in Japan.
During the final warehouse fight a lookalike of Jean-Claude Van Damme appears and is quickly dispatched by Hoshino. This was done as a joke about the supposed rivalry between the real life actors and Don Wilson's repeated challenges to Van Damme to fight him for real.
Originally, Chris Penn had been set to play Hoshino's L.A. partner; however, he was doing another movie, so the script was changed to cast a female partner (Farrell), bringing a love story element to the film.
An HBO premier.
Hoshino is actually his Japanese mother's family name which is why Don used it in Japan as his professional name.