Of course, the real cameras were also rolling, including the heavy 3-D rig of South African Julian Boulle -- "the best wingsuiter in the world," Holmes said -- who can fly in excess of 100 mph while keeping his subjects center frame. Boulle filmed Holmes as well as three members of the Red Bull Air Force, Jon DeVore, Andy Farrington and Mike Swanson, while torpedoing between buildings on lines chosen by Holmes, some of which featured mandatory turns.

"It was surreal," Holmes said. "I could actually see inside the buildings through the windows, like, oh, that guy's desk is messy."

The July stunts were meticulously planned and executed with cooperation from the city of Chicago. Mayor Richard Daley spent time on the set, and explosions echoed through the streets like mid-morning bombs.

The wingsuiters' role came about after director Michael Bay -- a Hollywood titan who directed "Armageddon," "The Rock" and "Pearl Harbor" -- watched Holmes on "60 Minutes" in the fall of 2009. Bay was fascinated by what he saw and called "Transformers" producer Steven Spielberg to see about using wingsuits in his next film.

According to Bay's comments in a Chicago Sun-Times article last summer, the jumps are likely to be confined to a short but raucous end scene involving aliens. Yet for Holmes and the other birdmen, chances like this don't come along more than once or twice in a lifetime.

"It was very technical, technical flying," said the 31-year-old Holmes, who estimated he's BASE jumped and wingsuited more than 1,000 times each. "But it was incredible. It's every BASE jumper's dream to fly in a city like that."

He explained: "When you're flying next to a cliff, it's not perfectly sheer. There are bushes sticking out, rock ledges. But with a building, it's straight down, totally sheer. So we were able to fly very close, safely. There were times when the building was two or three feet away from my left hand."

In addition to seven helicopter jumps, Holmes and his stunt mates plunged off the Willis Tower four times and the Trump Tower twice, as many as five birdmen at once. By the time they shot their scenes, they'd already completed extensive training, highlighted by a trip to Lauterbrunnen, Switzerland, where they scouted then leapt off 900-foot cliffs to familiarize themselves with each other's habits and to test equipment.

"We did hundred of jumps together to prepare for this, because we were scared," said Holmes. "There was a huge amount of trust building to be done. Mike, Jon, Andy, Julian and I needed to be able to anticipate each other's moves with little to no communication. The guys following me needed to trust that I would not fly them into a building."