Choi Min-sik trained for six weeks and lost twenty pounds to get in shape for the role of Dae-su, and did most of his own stunt work.
(at around 29 mins) Dae-su Oh bangs his head on a table right after stopping Mi-do from reading his diary. This was an unscripted accident, likely causing Choi Min-sik real pain. However Kang Hye-jeong (Mi-do) kept her cool and continued on with her lines. The director, in an interview, said that he kept and used this supposedly NG ("no good") scene for its comical and emotional value.
(at around 1 min) Scenes at the police station with drunken and disorderly Dae-su Oh were the very last scenes the director scheduled to film. Choi Min-sik (Dae-su Oh) ad-libbed most of these "drunken" scenes, including the scene of him playing with toy wings that he bought for his daughter. He also ad-libbed many of his lines during the penthouse scene, including the anthem of his school.
(at around 8 mins) When talking about sleeping gas used by Russian special forces, Oh Dae-su refers to 2002 Moscow 'Nord-ost' theater hostage crisis.
The recurring painting on the wall of Oh Dae-su's confinement complex is "The Man of Sorrow" by Belgian painter James Ensor.