By Geoff Boucher, Los Angeles Times staff

On the last day of postproduction on " Iron Man 2," director Jon Favreau looked like a broken man. "You don't want to shake hands, I'm sick," said the filmmaker whose superhero blockbuster just might be the movie that finishes as the highest-grossing release of 2010. But that possibility was far from Favreau's mind.

"I feel like I'm finishing this one the way El Cid finished the war, strapped onto his horse by his men and sent into battle dead."

One reason the battleground was so rough on Favreau is the secret weapon that made the first "Iron Man" such a nimble and memorable movie -- namely Robert Downey Jr. as the titular hero and Favreau's intense commitment to capture his singular spark on the screen. Like the first film, "Iron Man 2" was essentially rewritten over the entire filming process. The large set pieces, which required months of visual-effects work, were locked in and became the solid brick scenes of the film, but every bit of mortar between them was available for improvisation. Downey would try a dozen approaches and follow the one that worked. That meant a relentless need to patch, rework and rewire entire chunks of the film.