In the original script when Nick Chen walks in on Henry Vu being fed grapes in the bath, she was supposed to be giving him oral sex. Yun-Fat Chow said that Chen would not continue a relationship with May after witnessing this act, and the grape-feeding was suggested.
The close-ups of Yun-Fat Chow shooting a criminal at point blank range with a small revolver (as in the opening, and massage parlor raids) were directly inspired by the famous photograph of a Vietnamese soldier committing an execution.
The trivia item below may give away important plot points.
In the bathroom scene, where Yun-Fat Chow reacts to finding out that Mark Wahlberg is working undercover, the script called for him to break mirrors and knock a hand-dryer off the wall. This was decided to be an American response with physical violence, and the breathing technique was used.