"It's small," Fox, 24, told MTV News of her character's waistline. "We got it down to 18 inches. But in the action scenes, we loosened it up, because everyone was afraid I was going to pass out."
In the flick, which opens June 18 and is based on the DC comic of the same name, Fox portrays Lilah, a prostitute who packs heat in both pistol and physical form.
Her impossibly tiny waist came courtesy of vintage corsets, which the Emporio Armani Underwear model admits left quite an impression -- and not in a good way.
"At the end of the day when we would take it off, it would leave these deep grooves, these deep indents from the boning in the corset," she told MTV News.
"I'd have it for the whole next day. The next 24 hours I'd be imprinted with the corset. It hurts your ribs!"
Yet somehow Fox managed to find some pleasure in the pain and is now a fan of the uncomfortable underpinnings.
"I do think there's a place for corsets," she said. "They should make a reappearance. I enjoy them, but maybe I'm sick."
In related news, find out who won the 2010 Undie Awards!