Due to his smart aleck improv genius, Robert Downey Jr., as Tony Stark / Iron Man is my all-star pick of the current worldwide box office smash “The Avengers.” In this accompanying video, he talks about what ingredients makes the Marvel Comic Book adaptation such a success. One question: Would Edward Norton have done a better job as Bruce Banner than star Mark Ruffalo? Whatever. Downey Jr. works well with him, and he makes anybody acting opposite of him look good.