Zack Sharf Digital News Director Christopher Nolan appeared on “The Late Show” for a lengthy discussion with Stephen Colbert in which the filmmaker heaped praise on his “Oppenheimer” star Robert Downey Jr., who is widely considered the Oscar frontrunner for best supporting actor. Nolan said he always wanted to work with Downey because of his “generosity” and “charisma.” The director went on to call Downey’s role as Iron Man, which birthed the Marvel Cinematic Universe and carried it for a decade, one of the most important casting decisions ever made. “With anybody you work with, with actors…you’re looking for some kind of connection emotionally, empathetically,” Nolan said about being drawn to Downey.