He had already shaved his head, Brynner-fashion. He was worried that if he didn’t the audience might “hold back their feelings,” he said, and he hoped that this way he might lure them in. “It’s like come, come, and then you’ll get pulled into the maze,” he explained.

Mr. Sher, who is a tireless fusser and tweaker — and who believes that just getting an entrance right can explain a great deal about who a character is — had already discarded his work from the day before. Instead of opening the scene the way it’s usually done, with the king seated on his throne, he wanted Mr. Watanabe, attended by a kowtowing retinue, to walk the length of the Beaumont’s thrust stage.

Mr. Watanabe obliged with a stroll that was both regal and businesslike, and when he stopped to hear that the French had just colonized Cambodia (dialogue that Mr. Sher discovered in the original script and has restored to the show), he registered kingly alarm and annoyance at those predatory Europeans.

Shuffling across the rehearsal hall in stocking feet, arms at his side, his hands out like landing flaps, Mr. Sher nodded his approval. When he was out of Mr. Watanabe’s hearing, he said: “When we first started working, I thought, ‘Oh my God, this is maybe one of the best actors I’ve been around in my life.’ So many choices, so many things he can do. He’s got so much capacity for so many different kinds of work that it’s off the charts.”

In casual conversation, Mr. Watanabe can more than hold his own in English, but he likes to have his assistant and translator, Satch Watanabe (no relation) around just in case. Over lunch one day at a Japanese restaurant on the East Side, he explained that he grew up in Koide, Japan, a mountain town he compared to Denver, and that as a boy he studied the trumpet, hoping to become a musician. But when he was 13, his father, a calligraphy teacher, became ill, and there was no longer enough money for music lessons.