The Hollywood actor has revived the superhero for an upcoming sequel, which will see his character pitched against villainous Whiplash, played by Rourke.
And Downey, Jr. was fascinated by his co-star's unusual acting technique, because Rourke used photographs of his beloved Chihuahuas to help him portray tearful moments.
He tells Britain's Daily Express newspaper, "He would have someone holding pictures of recently deceased pets off camera to make him feel sad or whatever for a scene. It was all serious stuff. I've never seen anything like it."