According to Scott Glenn, the original script was about a ruthless club fighter from California with no family and no real background who gets involved in bringing a sword to Japan, and through a lot of crazy adventures he winds up with a martial arts sensei. The core of the movie would have been about father and son finding each other from completely different cultures. After shooting started, Glenn became aware very quickly that all those character-driven scenes were either being cut or shortened to almost non-existence, and that he was doing a martial arts movie. Toshirô Mifune came to him and told him: "Look, this is what's happening. I'm disappointed, and I know you are, but this is what it is. So you can either have your heart broken every day, or you can use this experience as an opportunity to be spending an interesting time in Japan with me as your tour guide." Glenn accepted Mifune's offer.
Toshirô Mifune was a student of such martial arts as aikido, kendo, karate, and kenjutsu, as well as being an excellent handler of Japanese samurai swords.
Much of the movie was filmed on the grounds of the ancient Sokokuji Temple, which had remained unchanged for seven hundred years.
According to co-writer John Sayles, director John Frankenheimer brought him to Japan to change all the Chinese characters of the script into Japanese characters in only five days. One day he went to Kyoto to see the locations for the big battle scene at the end of the movie; the other four days he was locked up re-working the script in the Imperial Hotel in Tokyo while Toshirô Mifune took everyone else out for dinner.