Actor Allusion: Philip Chan, who played Superintendant Pang, was a CID cop in the '70s and for a while was the head of one of the first undercover units of the Hong Kong Police.
Trash the Set: The teahouse that served as the setpiece for the first big shootout of the film had been slated for demolition, and John Woo got permission to use the teahouse for the shootout in question.
What Could Have Been: Hard Boiled was intended to be less stylized and more gritter, akin to a Dirty Harry film, with Tony Leung Chiu-Wai picked to play a psychopath poisoning baby formula. Most found this idea too disgusting, and a new plot was written with Tony Leung Chiu-Wai playing an undercover cop. A number of story elements were improvised during filming and not part of the script, such as the character of Mad Dog.