First film in history to have a sex scene with sign language.
All characters that used sign language learned alongside deaf and nonverbal/mute individuals, having sign-language conversations with them about the movie.
(at around 45 mins) Song Kang-ho shouted for three days to break his voice in order to sound more authentic and tired for the scene where he is screaming for help through the black plastic bag.
Originally director Park Chan-wook wanted the colors to fade to black and white as the film progressed. Budget constraints prevented the color desaturation of 'Sympathy for Mr. Vengeance' but Park would revisit the idea of color desaturation with the Region 3 Special Edition release of Lady Vingança (2005), which included a director's cut where the colors fade to black and white during the film's progression.
The script was finished in 20 hours of non-stop work.