Don't mess with the action. Directors try to avoid this when filming, as fights, chases and other stunts require considerable advance planning.

So director Nima Nourizadeh was pleased when he realized he needed to change the first fight scene in his new film, "American Ultra," and his lead actor was so well-trained he could just roll with it.

"It was a cool fight but it was way too long," Nourizadeh recalls. "Now it's like 10 seconds -- we want people to be shocked, like if a friend was suddenly able to break out in a way you've never seen before. It's about Jesse, suddenly activated, springing into action."

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The Jesse of this action picture, of course, is Jesse Eisenberg and --

Wait.

Rewind the tape.

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Eisenberg? In an action movie?

Oh, sure, there was "Zombieland," where he toted a shotgun (and Purell). But in "Ultra" he demonstrates some serious hand-to-hand combat. And more.

The film opens Aug. 21, and it's that kind of least-likely casting that fuels the outlandish premise here -- that a secret army of mind-controlled, super-soldier, sleeper agents live among us, and they could be anybody. Seriously, anybody. Even the stoner dude at that sad all-night convenience store in West Virginia.

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Eisenberg plays that stoner, Mike Howell, alongside Kristen Stewart as his girlfriend, Phoebe, plus Connie Britton and Topher Grace as feuding CIA operatives and John Leguizamo as a colorful small-time hood.

"The movie is so unusual," says Eisenberg. "It's funny, sweet, romantic, full of hyperkinetic action. It's a mash-up of all these interesting tones."

Not your typical action-adventure. And not your typical Eisenberg.

"Ultra" is tough to categorize. It morphs from stoner comedy . . . to romance . . . to rock 'em, sock 'em action flick.

"That's why I was attracted to the script," says Nourizadeh, a British director known mostly for commercial and music video work, till his 2012 splash with "Project X." That film -- a comedy about a high school party run amok, shot with those handheld shaky-cams that usually trigger more nausea than nuance -- was a hit. This second feature should prove "he's the epitome of cool," says Grace. And not some one-hit wonder.

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It's also the second completed feature for screenwriter Max Landis, a rising star who debuted with the 2012 high school sci-fi thriller "Chronicle." ("Victor Frankenstein," his revamp of a classic, starring Daniel Radcliffe and James McAvoy, hits in November.)

Nourizadeh loved the unpredictability of Landis' writing, like the scene in which Eisenberg and Stewart lie together in bed -- he's feeling vulnerable, and she comforts him. It's quiet, touching, intimate.

"Then he blows a raspberry on her neck," says Nourizadeh. "You're always being taken in one direction, then flipped around another way. It never felt obvious."

"There aren't many movies like this," Grace agrees. "An action film that isn't a sequel, a remake, a reboot." Or based on some comic strip? "Exactly," he says.

This fresh take on a familiar genre may be due to Nourizadeh's background in music videos. Or so Eisenberg suspects, having done a fair share of "intimate, talky movies," he says, where the emphasis is more on mood than unusual sequences or shot composition.

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Here, an odd, unexpected beauty creeps in. The screen is awash in rich colors and eye-catching images -- like a spoon on black pavement, or a cup of ramen noodles seen from above, a dazzling swirl of gold and amber, like one of those recent NASA flyby shots of Pluto, with flecks of corn and the occasional green pea.

"My production office was wall-to-wall images," says Nourizadeh, describing the inspiration photos lining his walls. "It did kinda look like a mad man's room."

The cumulative effect of all these shots? "It makes the movie look and feel really special," says Eisenberg.

So does the sight of him grappling with some thug.

Eisenberg and Stewart were last paired on screen in "Adventureland," the 2009 comedy by Dix Hills native Greg Mottola, inspired in part by the Farmingdale amusement park. In "Ultra" the two come off darker, tougher, not just because of the grungy wardrobe and stringy hair, but the way they throw punches.

Eisenberg found the stunt work exhilarating. For months before the shoot, he spent weekends with stunt coordinator Robert Alonzo learning martial arts disciplines like pencak silat (from Indonesia) and muay Thai (Thailand).

"It allowed me to free myself of nervous distractions," says Eisenberg. "I became fully invested in my character's plight because I was doing something physical. I loved it."

It was also kinda cool, he admits, to learn how to kick butt.

Correction -- the goal in such pursuits is not merely (or always) to cause bodily harm. With their emphasis on balance and mental focus, "they're somewhere between exercise and fighting," says Eisenberg.

"The three months we filmed, I'd never been stronger," he says. "Then the shoot ended, I went on to do something else and immediately deflated."

He laughs.

"I wish they'd make a sequel -- if for nothing else than for me to work out again."

GREAT SLEEPER FILMS

An FBI investigation unearthed 10 undercover agents in the U.S. working for Russia in 2010. But Hollywood -- on both the big and small screens -- has found way more than that.

THE MANCHURIAN CANDIDATÉ(1962) The original and best version of brainwashed American prisoners-of-war and a wicked Angela Lansbury.

TELEFON (1977) KGB agents Charles Bronson and Lee Remick try to thwart a plot to activate Soviet sleeper agents across the U.S. -- brainwashed citizens who don't even know they're spies.

EYE OF THE NEEDLE (1981) Donald Sutherland and Kate Nelligan star in this suspenser based on the Ken Follett novel about a German spy in Britain during World War II.

NO WAY OUT (1987) Another Cold War drama with Kevin Costner hunting an alleged KGB spy in the U.S. government.

LITTLE NIKITA (1988) Again with the Soviet spies? This time it's Mom and Dad; teen River Phoenix gets the 411 from FBI guy Sidney Poitier.

THE AMERICANS (2013) KGB officers Keri Russell and Matthew Rhys impersonate a typical married couple in 1980s Virginia. (The FX series' fourth season airs next year.) -- JOSEPH V. AMODIO