“Hey, what’s your name?”

“Arthur.”

“Hey, Arthur, you’re a really good dancer.”

His arms float overhead to form something like a diamond crown. “I know.”

“You know who’s not? Him.”

He aims his gun and fires at the imaginary him. The bullet hits a wall and in that moment, Arthur is both alarmed and exhilarated: Dance is his path to bravery, something he’s never known. As Arthur recedes and the Joker takes over, the choreography becomes more drawn out. In the transformative bathroom scene, panic morphs into an eerie power. Mr. Phoenix softly crosses one foot over the other and twists, curling his arms overhead and around his torso. His shoulders hike up and his elbows jut out dangerously as his body ripples and swells until, in the final moment, his arms extend to either side. This is the Joker’s power pose.

Sometimes Mr. Phoenix, who lost a great deal of weight for “Joker,” has the look of a ballet dancer on a break from rehearsals. Pale and gaunt with wavy hair pasted to the sides of his face, his appearance, at times, has a touch of Rudolf Nureyev or Sergei Polunin — two Russians with attitude. His skin stretches tautly over muscles and protruding ribs. But it’s not just a cosmetic transformation. Nor is what he does ballet. Mr. Phoenix has the sinewy ability to turn his body — particularly his back — into a Butoh horror show of odd, freakish angles.

But more than Butoh — the postwar Japanese form known, in part, for its dark, slow-motion movement — his dancing embraces vaudeville . That makes sense. Growing up, Mr. Phoenix spent time busking with his brothers and sisters in Los Angeles; vaudeville is in his body’s history, too. And while he told The Associated Press that Ray Bolger’s “The Old Soft Shoe” was an inspiration for the hubris of the Joker, there’s also something of Astaire in his movement , especially in the way he creates lightness and space in his upper body.

Yet Mr. Phoenix’s dancing also feels fueled by sensation , as if he were delving into Gaga, the movement language created by the Israeli choreographer Ohad Naharin.