The weapon Raizo favours throughout the movie is the kusarigama, a notoriously difficult weapon to master.
After Raizo (Rain) is captured by Interpol and chained to the wall, one of the agents looks at him and says, "He looks like he should be in a boy band." Rain started his career in the boy band Fanclub.
Producers Lilly Wachowski and Lana Wachowski hired J. Michael Straczynski to completely rewrite the script six weeks before filming started, as they weren't satisfied with Matthew Sand's original work. Straczynski finished the rewrite in 53 hours, and was studio-approved and sent to actors the same week.
Korean pop star Rain had no martial-arts experience prior to filming. He trained extensively for six months in martial arts and weapons handling with stunt and action choreographers Chad Stahelski, Peng Zhang, Jon Valera and Jonathan Eusebio as well as weights and cardiovascular workouts with personal trainers; he also went on a diet of chicken breasts and vegetables.