While filming, Tatsuya Nakadai was afraid during most of the sword and spear fighting scenes because real swords were being used, a practice now forbidden in Japanese films. His concern was not alleviated even though professional swordsmen were employed during the choreographed swordplay.
Stage-trained actor Tatsuya Nakadai and older film actor Rentarô Mikuni could not agree on an acceptable speaking voice while sharing the film stage. Nakadai spoke loudly and Mikuni spoke softly each citing their related acting experiences for their choice. They strongly disagreed with each other. The director, Masaki Kobayashi, halted filming and stated that he would not resume until both the actors came to an agreement. They did; stopping the shooting for three days!
Seppuku and harakiri (the US working title) both mean to commit ritual suicide in Japanese. However, seppuku is the formal term, derived from the kanji characters for "hara" (belly) and "kiri" (cut); harakiri is the cruder, less polite term for this act.
The clan featured in the film, the Ii (sometimes spelled Iyi) have produced one of the most important and controversial figures of modern Japanese history: Ii Naosuke, Chief policymaker of the Tokugawa Shogunate best known for signing the Harris Treaty with the United States, granting access to ports for trade to American merchants and seamen and extraterritoriality to American citizens.