It's not very often that you see the screenwriter--that notoriously low man or woman on the totem pole--on the set of a big-budget Hollywood movie. It's even less common to see the screenwriter there on a daily basis, conferring with the powers that be. The production of Iron Man, however, is different, with screenwriter Mark Fergus in constant attendance.

Just what is he doing there? Director Jon Favreau has allowed his actors--Robert Downey Jr, Terrence Howard, and Jeff Bridges--a certain improvisational freedom. But should they venture too far beyond the boundaries of their established characters and Marvel Comics lore, Fergus is there to reel them back in. And perhaps add a few new lines.

The idea, says Fergus, is to prevent "too much drift." He elaborates: "I'm not there to defend what I've written. You have to be open to creative input. That's part of being a screenwriter. Robert and Terrence will come up with things--but we want to be sure that whatever they do works for the character and the Marvel universe."

The initial process of getting Iron Man on down paper was a "fantastic collision of ideas," says Fergus, who cowrote the screenplay with Hawk Ostby. (The two also collaborated on Children of Men and the forthcoming John Carter of Mars.) "We have a process where we write all of the elements we like first," he explains. "The first and second drafts we wrote on our own. We didn't look at other's drafts. We didn't talk about it. We wanted to get everything we loved down on paper before there was a collision of ideas. Then we would meet and compare notes. Some things we'd keep, some we'd trash."

In the transition from comic-book page to big screen, says Fergus, the character of Tony Stark is relatively unchanged. And this superhero has his issues: "He's trying to come to terms with himself, personally and emotionally. It's a breakdown and rebuild. In a way, the film is about a hero who doesn't always make the right decisions."