When he was created by Stan Lee in the early 60s, Tony Stark was conceived as a character you are not supposed to like. Stark is a rich, right-wing arms dealer. He is everything the young core audience of the comics industry is meant to despise. Lee created him as a challenge for himself to force that audience to like Stark anyway. He did it by making Stark a decent person with a generous heart. Downey, director Jon Favreau and the people writing the Iron Man feature film, however, took a different path: they made him a fun drunk.
That was something that the mainstream Tony Stark never was. He was a drunk, yes, but hadn't been one consistently for nearly 30 years. The element Downey used was borrowed from Marvel's "Ultimate" line, an alternate rebooted universe born in 2002 that wasn't bogged down by continuity. But here's the problem - that Tony Stark is a dick. He intentionally gets loaded before stepping into the Iron Man suit (quipping that no one would get into that thing without having a few drinks.) He has no moral centre, and if he's going to be the lead character of a motion picture franchises, that just won't fly whether it has jet boots or not. There had to be an element that made Stark human. Fortunately, Marvel moviemakers had a blueprint for that which had recently been released in 2005.
That's where Warren Ellis and Adi Granov's "Extremis" enters the picture. A reboot of sorts, it retcons Iron Man's origin so it takes place in Afghanistan instead of southeast Asia. It makes him a little bit less of a CEO and more of a garagehead, an element that was DEFINITELY at the forefront of the motion pictures. It also makes Stark much more ambitious, placing a lot more emphasis on his attempts to improve the world outside of punching supervillains. The most crucial character scene comes in the first issue, where Stark is interviewed by an aggressive left-wing documentarian. During it, Stark laments that he had hoped his weapons program would fund positive world-changing projects. However, he admits that he's unsure if that's the case.
THAT Iron Man makes up the other, more important part of Downey's portrayal and had a huge influence on the first movie. Now we're at number three and the rest of what happens in "Extremis" is set to play a role.
Let's get to that: the book is titled after the Extremis science project, a technological update of Captain America's super soldier serum. Once injected, it completely re-writes the subject's DNA in a manner they see fit. After Stark gets his ass kicked by the first "test subject," he undergoes the process himself, reemerging with the ability to control the Iron Man suit with his thoughts. Stark emerges with a new sense of confidence and - surprise - the good guy wins.
It's unclear how much of the story will factor into the third movie, though. The two Extremis scientists - one who exits quickly in the comic - are in it. But so is Mandarin, who wasn't a part of the story at all. It's also unclear if Stark undergoes the same kind of drastic procedure. Either way, the most important part of this book - Stark's character - was gleaned from "Extremis" for the first film and if anyone who loves the movies asked me for an Iron Man trade to read, this is the one I would suggest.