An actor prepares. Charlize Theron goes to the grassroots.

For acclaimed turns in "Monster" and "North Country," the dedicated actress spent weeks studying the methods and motivations of a murderous Florida prostitute and sexually harassed Minnesota mining women.

For "Aeon Flux," she studied a gecko.

Yes, gecko -- as in tropical lizard.

"There's a scene where Charlize runs on all fours, and her belly's only 3 or 4 inches off the ground," says stunt coordinator Charlie Croughwell, "so we watched National Geographic videos of geckos to analyze their movement."

Says Theron: "I'm from Africa. I know all about geckos. I used to play 'Alcatraz' with them in my yard."

"Aeon Flux," which opens this weekend, is the big-screen adaptation of an animated MTV sci-fi series about a mysterious assassin who prowls the underworld of the futuristic city-state of Bregna.

But for Theron, getting cartoon-lethal meant more than dying her blond hair black. Big-bad Bregna meanies don't just keel over because you're drop-dead gorgeous.

"She had to look effortless but powerful, fluid yet ferocious," says Croughwell. "And we didn't want a lot of weapons. We wanted Charlize to get into a guy's face before she killed him."

Three months before filming, Theron began training in L.A. with Croughwell, whose workout regimen incorporated trampoline, hand-to-hand Krav Maga combat techniques and the Brazilian martial art of capoeira.

To teach her the dance-derived skills of capoeira (pronounced capo-edda), Croughwell called in master teacher Neal "Xingu" Rodil.

"Charlize was all-in, she had a lot of drive -- and a strong kick," said Xingu, 27, who started Theron out with basic maneuvers like an au' (cartwheel) or bencaou' (push kick). "She used to be a ballerina, so it came naturally."

At the outset, Theron still carried some of the 30 pounds she'd put on for her Oscar-winning role in "Monster." But after three months of grueling, four-hour workouts -- plus trampoline training with Cirque du Soleil's Terry Bartlett -- Theron was ready for her close-up macaco (monkey flip).

It's in the film, when Theron sneaks up on a guard, springs into a one-armed cartwheel, scissors her legs around the goon's head and spins down to the ground, snapping his neck.

"I'd practiced that hundreds of times but never quite nailed it," Theron says. "On the day of shooting, we had a stunt person try, but after maybe 15 times she still couldn't get it. I looked at Charlie and said, 'I'm ready, let's do it!' Then I nailed it five times in a row."

During filming, Theron herniated a disc in her neck after landing awkwardly on a back flip, but she returned after a six-week production delay and continued to do most of her own stunts.

"Charlize isn't the type to complain," says Xingu. "No matter how difficult the maneuver, she'd keep saying, " 'Let's do it again.'"