Robert Downey Jr went from nearing obscurity to this massive role that would change the course of his career. He plays Tony Stark perfectly. What makes him such a great character is how flawed he is. That's the problem with many comic book heroes. If they do have flaws, the flaws don't make for strong character choices. Tony Stark is beyond flawed. He's a maniac. He's a jerk. He's a troublemaker. But he also has the capacity to care and save lives.
Jon Favreau directed this one, and also placed himself into the Marvel Cinematic Universe as Happy Hogan. That's what I'd do, but I'd make sure to write myself in as a character who was guaranteed to be in every movie. An immortal character. I gotta get paid, am I right? I'm interested in job security.