"Infernal Affairs" was inspired by the film "Change Face" directed by Wu Yusen. Mai Zhaohui believes that the "face exchange" in "Change Face" is impossible in reality, so Mai Zhaohui asked whether it is possible to tell a story and exchange the two characters, but it does not require surgery, just identity Exchange with the heart. Mai Zhaohui and Zhuang Wenqiang have been communicating with each other in writing when they were creating the "Infernal Affairs" script. Mai Zhaohui wrote a complete script and sent it to Zhuang Wenqiang. Zhuang Wenqiang changed the script and wrote to Mai Zhaohui for revision.
In Mak Siu-fai's screenplay conception, there were initially only the story lines of the police going to the undercover underworld. Later, Liu Weiqiang added a lot of elements to the original script, and joined the underworld people to go to the police station as another line of undercover. In the process of revising the script, the positioning of "Infernal Affairs" also changed. At first, the script of the movie focused on the wit of the police and the bandits. Later, the elements of the gangster or hero movie became stronger.
