Chad Stahelski had a wild idea. The director of “John Wick: Chapter 3—Parabellum” dreamed of opening his new movie with an epic fight scene inside the New York Public Library starring Keanu Reeves and a character who ideally would be a cross between Andre the Giant and Kareem Abdul-Jabbar. The problem was that he couldn’t be sure this person of his imagination actually existed.

“Someone with a heart and soul who was fun and interesting and yet gigantic,” said the director. “I couldn’t find anyone.”

But...