There were a lot of memorable moments at the 2011 Hero Complex Film Festival, but nothing could compare to the firecracker finale on Sunday night when “Iron Man” star Robert Downey Jr. stunned the sellout crowd at the Chinese 6 Theatre in Hollywood with an unannounced appearance.

On stage, I was talking to director Jon Favreau about his star and their memories of the career-changing 2008 opening weekend of “Iron Man“ when the filmmaker reached into his pocket to pull out his phone. “I’m going to call him. Do you mind? Let’s get him on the set of  ‘Avengers’?” It was all part of planned bit where he would call the actor who played the title hero and start a phone conversation that would lead to a speakerphone chat and then, finally, the emergence of Downey from the high rear doors of the stadium-seating auditorium. It didn’t go quite as planned — maybe the signal was bad, maybe Downey just missed the call — but the actor has made a pretty decent living out of improvisational moments and inspired crowd-pleasing.

“Why call me? I’m here right now,” Downey said as the crowd’s confusion gave way to a thunderclap ovation. The two-time Oscar nominee joined Favreau on stage and the two embraced. There had been plenty of rumors in Hollywood that the relationship between the actor and filmmaker had turned frosty after the successful but grueling “Iron Man 2″ production, but if any of those whispers were true they also appear to be clearly outdated. The lively, smart, warm and intensely funny on-stage rapport between the two made it clear how their collaborationscould deliver the billion-dollar “Iron Man” franchise.

Favreau explained the choices that were made as far as tone and humor in the first movie and the key component of Downey.

“I always figure inevitably, if there’s a laugh to be had, we’ll find it, and if it doesn’t blow the reality, we’ll go for it, and especially for the casting of Robert Downey Jr. He’s just really the heart and soul of the whole thing. … It was very difficult to cast him, that was not who this new studio felt they wanted to necessarily brand themselves with as their main guy. He hadn’t really proven himself that he could draw audiences. But we knew he was right. I remember I met with him, and I was like, ‘This guy, oh my God.  He looks great, his attitude’s perfect, and he was passionate about the role.’ From the moment we finally got to cast him — and he had to screen-test for it, and he did; he fought his way to get the part, he blew everybody away, they couldn’t make an argument against him. God bless him, he worked hard, and I told him, no, it’s not going to happen, and he said, ‘I’d like to keep fighting for it,’ and I said, ‘I’ll go as long as you do.’  He did it. They couldn’t argue with him. He did it for a song. From the minute he came on board everything changed. … His whole personality informed a realistic view of Tony Stark, not just because of the role as it’s written, but as so much is read into it because you really believe that Robert could have lived in this environment and grown jaded with the world of fame, and he just understands how to  move through a room  like a guy who’s been famous since he was a kid, because Robert has been. He’s been a star since he started, and he grew up in a family where his father was a director and around Hollywood and New York and the film scene. So that was a big, big part of the tone. And we just hit it off really, really well and made each other laugh. And he always liked to do things different, and I was comfortable with a certain amount of uncertainty.”

Both onstage and off, Downey exuded the confidence and whipsaw wit of Tony Stark, but there also seemed to be no end to his simple gratitude for life today. For a man who knows what the world looks like from a chair in rehab or from behind prison bars, the vantage point of 2011 is not something he’s going to take for granted.

“It’s the best thing that ever happened to me, I mean, workwise, you know,” he said of the 2008 career-reviving superhero film. “And personally too, because you know, Jon and I went on kind of this journey together that was all about trying to take something and kind of maybe re-invent a genre a little bit. It was just incredibly exciting. When you have nothing to lose, you sometimes take risks. You might as well really make it trippy.”