This role is for someone who is willing to die for the Takeda clan.
Akira Kurosawa made two films in color before 1980's Kagemusha -- 1970's Dodes'ka-den and 1975's Dersu Uzala -- but neither of them were on the scale of this grand epic. Set in the 16th century during a period of great upheaval, the film is not unlike Ivan Reitman's Dave in that it's about a commoner who's tapped to double for a warlord, only for the charade to become more permanent when the warlord is felled (by a sniper's bullet instead of a heart attack while sleeping with his mistress). The key difference is Kagemusha is three hours long and Dave comes nowhere near that. Dave doesn't have massive battle scenes featuring thousands of extras in brightly colored armor, though. This is Spectacle with a capital S, which Kurosawa's old friend Ishiro Honda -- credited here as production coordinator -- helped him realize.
Tatsuya Nakadai plays the double role of the warlord, Shingen Takeda, and his "shadow warrior" (the English translation of the title), with Tsutomu Yamazaki as Shingen's brother, Nobukado, who's doubled for him in battle and puts his stand-in through his paces, and Ken'ichi Hagiwara as Shingen's ambitious son, Katsuyori, who's still smarting about being passed over in the line of succession. Nakadai delivers the key performance(s), though, with his nameless imposter rising to the occasion in ways Shingen's retainers couldn't have anticipated. After he's found out, though, he's relegated to the sidelines, a mere spectator as the final battle takes its bloody toll on the clan he was the (figure-)head of. Definitely one instance where having a front-row seat is not a plus.