NEW YORK — Kristen Stewart laughed loudly when asked about the action in her new film with Jesse Eisenberg. “It was so hilarious. Here’s Jesse, who in real life is such a non-gun person and a total pacifist, having to play this sleeper super-agent killing machine. But he did it great!”

A quiet chuckle followed from the always gentle, soft-spoken Eisenberg, sitting next to her in a Manhattan hotel room, who nodded repeatedly as his co-star described his true, off-screen persona.

“But that’s exactly why I wanted to do it,” said Eisenberg, who is best known for such films as “Social Network” and “Now You See Me.” “When I read the script I loved how this movie starts out as a stoner romance and then morphs right into this kick-ass action film with all kinds of stunts, fights, secret government agency intrigue and mayhem. I had never done anything like that.”

“That was true for me too,” agreed Stewart, who explained that “this is why we are actors. We get to play in this amazing playground of storytelling. …

“It gets a bit boring doing the same thing over and over again. I know something about that,” she added with a sly smile, apparently referring to her career-making role as Bella Swan in the “Twilight” saga.

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In “American Ultra” (opening Friday) Stewart plays Phoebe, the watchful girlfriend of Eisenberg’s absent-minded Mike character — a perpetually stoned guy working in a dead-end job in a West Virginia convenience store.

Abruptly, we discover that Mike actually is a genetically altered super-soldier/agent — part of a discredited program to create uber-assassins. Now, the government honchos have decided it’s time to kill off people like Mike to eliminate any evidence of wrongdoing.

That leads to quite the fast-paced romp as Mike and Phoebe work to keep themselves alive in the face of an army of U.S. operatives bound and determined to bring them down.

Since Eisenberg truly is a mild-mannered, always polite kind of guy, one had to wonder how he felt when he had to portray such a ruthless killer — once he fell under attack and his assassin skills were activated.

“You know, it really was kind of cathartic in a perverse way,” Eisenberg said with a laugh. “I guess I was able to tap into some deep-seated rage that I didn’t even know I was capable of expressing. After we did some of the nastiest killing scenes — covered in so much fake blood — I actually felt kind of good. I’m sort of embarrassed to admit that. But it was true.”

Stewart smiled and said, “And you did it so convincingly. It was funny watching you do all that!”