And the story of Plummer, Shatner, and
Henry V is awesome, if you've not heard it.
Plummer was a bit notorious with the ladies, and a vigorous one-night stand somehow dislodged a kidney stone in Plummer, leaving him in excruciating pain so intense that he was hospitalized and doped to the gills with morphine. When people from the Stratford Festival let Plummer know that his understudy Shatner would be performing in his stead, Plummer was determined to escape the hospital but he didn't even reach the door before 1) even more excruciating pain and 2) more morphine.
Shatner's performance was considered a triumph. Whatever Plummer had done, Shatner did the opposite. If Plummer had delivered a speech standing up, Shatner delivered it while sitting. If Plummer shouted, Shatner whispered. Shatner felt that this was the moment when he became an actor, and Plummer said, paraphrasing from memory, "I knew that son of a bitch was going to be a star."