Stone is filling a hole created by Uma Thurman, who initially was negotiating for the part.
Thurman backed out for unspecified reasons and Stone stepped in at the last minute.
Bill Pullman and Tony Hale are also late additions to the cast, which includes Stewart, Jesse Eisenberg, Topher Grace, John Leguizamo and Walton Goggins.
The film focuses on Mike (Eisenberg) and his live-in girlfriend (Stewart). He’s a slacker who lives in a small town until his past suddenly emerges to bite him in the ass.
Turns out he’s part of a government program that created highly trained warriors. Stone’s character spearheads the secret government program.
When she finds out government assassins are out to eliminate Mike, she swings into action to protect him “like a momma bear protecting her cub,” according to the film synopsis.
Stewart raised eyebrows last week when a New Orleans hairdresser posted a photo on Instagram showing Stewart with orange hair. She was getting into character for the movie.
“Project X Director Nima Nourizadeh will helm the picture, according to deadline.com.
Stone, 56, is seeing a resurgence in her career. She just finished filming a TNT pilot “Agent X” and is starring in Woody Allen flick “Fading Gigolo,” which stars Allen and Turturro.
Stewart, who celebrated her 24th birthday last week, also has a number of films in the works.
She’s already wrapped shooting on “Camp X-Ray” and “Sils Maria.”
Beside American Ultra, she’ll begin work on a sequel to “Snow White and the Huntsman” next year and she reportedly will appear in indie movie “Anesthesia,” filming in New York City.