About an hour into "Iron Man" comes a little moment, easy to miss, where Robert Downey Jr ., as brilliant-but-dissolute-playboy Tony Stark casually reaches in and slips his hand inside the hologram of the armored suit that will make him a superhero.
It's not a big visual-effects setpiece -- and this movie has plenty of them -- but it epitomizes what separates "Iron Man" and its visual effects from most superhero films: Downey improvised the bit on the set and the vfx were built around his performance.
"That's just Robert," explains vfx supervisor John Nelson . "He's such a gifted improviser. He affected everything."
Such a special performance also presents vfx artists with a special challenge: Get out of the star's way and let him shine.
That came up again in a sequence where Stark is trying out the components of his suit, aided by his not-entirely-reliable robots, but neither the suit nor the robots work exactly as he'd like. Downey turned the sequence into an extended slapstick.
"It was all discovered on the day," says director Jon Favreau . It took Nelson and the vfx artists the better part of a year to to match Downey gag for gag.
Favreau warned Nelson up front, "I am not a fan of CGI," and gave Nelson strict marching orders.
"Number one," says Nelson, "(the vfx) had to satisfy the story. Number two, it had to look real. And number three, he never wanted it to be too many shots. If it could be told in a more straightforward way, we would do it. (He) didn't want the movie to be spectacle for spectacle's sake."
Everybody understood that Downey was, in Nelson's words, "the heart and core of the movie," but Iron Man isn't always Downey in a metal suit. Sometimes he's a stuntman. Often he's entirely computer-animated, as when he has a dogfight with a pair of F-22 fighters that mistake him for an enemy weapon.
To ensure the CG hero would be as lifelike as possible, Nelson sought out Hal Hickel , Industrial Light 's animation supervisor on the "Pirates" franchise, to take the same role on " Iron Man."
Favreau remembers haranguing Nelson and ILM about a shot where he thought Iron Man wasn't real enough. But he shot turned out to be the live-action Iron Man suit, not CG. "When I knew they had me fooled I knew it was OK to be in the film," Favreau says.
Another challenge came every time Downey slid on the Iron Man armor, removing him from the screen. The solution was shots of Downey interacting with the heads-up display inside the suit.
"When he would glance left we would drive in a graphic from offscreen left to meet his gaze," explains Nelson. Downey's "unspoken dialogue" with the HUD puts him back on the screen with visuals more interesting than a disembodied talking head.
After a year-and-a-half laboring on "Iron Man," including effects for a couple of aerial duels, combat action in Afghanistan and the intricate showdown with the main villain, Nelson still loves talking about moments like Downey's hand-in-glove bit.
"I'm as proud of that small stuff as I am of the big stuff," he says. "Certainly the big stuff has to be there But the attention to detail, that's, I think, why the movie plays so high across the board. Everything feels right and fits."