In 1980 the Japanese director Akira Kurosawa directed the film "Kagemusha" (The Shadow Warrior). In Japanese the word Kagemusha means political decoy. The film is a work of art, tells the story of a humble man trained to impersonate the Takeda clan leader, the "Daimyo" Shingen.
Here we see when the double is presented to the general. The double looks so much like the Daimyo, and is so well trained to imitate his gestures, that all his generals, including his son and heir Katsuyori, think the double is the real Daimyo until the truth is revealed:
Kagemusha is introduced to the generals
The Kagemusha, or double, is not considered very useful until the Daimyo Shingen has the misfortune of being wounded during the siege of an enemy castle. When he realizes he´s mortally wounded he call his generals, including the heir (his son Katsuyori), and tells them he´s very sick and is going to die, but that his death should be kept a secret.
The Daimyo was very concerned that his heir wasn´t ready to take over the clan´s leadership. Knowing that his enemies would seize the opportunity to destroy the clan if they saw it weakened by an inexperienced leader, he decided to be temporarily replaced by his double until Katsuyori learned the job.
In this scene we see when the Daimyo Shingen, lucid but very ill, explaining his plan to his generals:
Dying, Shingen reveals his plan
In the film the two characters, the Daimyo and his double are the same actor. Therefore it´s not surprising to see they look and make the same gestures. I guess in real life, if this happened, the Kagemusha must have looked a lot like the general, because at that time there wasn´t plastic surgery. He also must have practiced the general´s gestures and his way of speaking to have done such a perfect job.
The film ends when the heir Katsuyori fired the double, ignored the advice of his generals, and attacked the rival clan at the wrong time, which lead to the defeat at the Battle of Nagashino.
Since the double was such a perfect actor, history can´t tell us the exact time when the double began his performance and when the real Daimyo Shingen died. The Kagemusha dies at the end of this battle along with Katsuyori and most of the Takeda clan´s army.
History tells us the victory at Nagashino allowed the great Tokugawa Ieyasu to win the Japanese civil war and to become Japan´s Shogun.
Battle of Nagashino by Nathan Ledbetter
Tokugawa Ieyasu´s Biography
Akira Kurosawa´s Criterion Collection Overview