Brad Pitt's character says that he never forgets a face but struggles to remember who The Wolf is. In real life, Pitt has prosopagnosia, a condition that impairs the ability to recognize familiar faces, including one's own face. He has said in interviews that, just as his character does, he needs to remember the context and the situation in which he has met someone in order to remember who that person is.
Director David Leitch and Brad Pitt had a long working relationship before Bullet Train (2022). Leitch has worked as Pitt's stunt double in multiple films including Fight Club (1999), Ocean's Eleven (2001), Troy (2004) and Mr. & Mrs. Smith (2005). Pitt also made a cameo in Leitch's second directorial effort Deadpool 2 (2018).
The scene where Lemon is catching The Son's head during the car chase was improvised as Logan Lerman pretended to be completely unconscious to make the scene authentic.