Beginning in June 2000, it took Bong Joon Ho a year to write the script for Memories of Murder (2003), yet he has stated that: "For the first six months, I didn't write a line of the script. I just did research."
In order to make his character Detective Seo look properly worn-out by the stress of the case, actor Kim Sang-kyung deliberately limited his food intake and slept fewer hours.
One deleted scene shows detective Cho walking in a red light district, and then getting a massage from a girl as he asks her about possible leads regarding the murders. The scene was shot in a real red light district in the city of Jeonju and featured real sex workers. They had to ask the local pimp for permission to film.
Many viewers mistook Kwok Seol-yung as detective Park's wife and also thought that they were at their home in the scenes where they are together. She is actually a prostitute and they are at a brothel.
The first jump kick and scuffle from Park was improvised by actor Song Kang-ho.