“Killers of the Flower Moon ”Cast Discuss Using 'Authentic' Osage Language in New Clip (Exclusive)

"We're making a film about a historical event that is central to the Osage history, so [it's] important we spoke that language," says director Martin Scorsese

The Killers of the Flower Moon cast and crew were dedicated to accurately portraying the Osage culture.

In a behind-the-scenes clip shared exclusively with PEOPLE on Indigenous Peoples' Day, director Martin Scorsese and stars Leonardo DiCaprio and Lily Gladstone discuss staying true to the language spoken within the Osage Nation.

"We're making a film about a historical event that is central to the Osage history, so of course it's important we spoke that language," says Scorsese, 80.

"We wanted to be as authentic as possible, and using the duality of both English and Osage was a symbol of the enmeshment of the two cultures," adds DiCaprio, 48.

Gladstone, 37, says "speaking Osage changed the way that [she] moved" as her character Mollie.

"It took months to get comfortable with the different pace of speaking," adds the actress, who has Native American heritage.

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Related: Leonardo DiCaprio, Martin Scorsese's 'Killers of the Flower Moon' Gets 9-Minute Standing Ovation at Cannes

"This is the 1920s, where people were still speaking the Osage language daily," notes Geoffrey Standing Bear, Principal Chief of the Osage Nation.

Touching on Robert De Niro's devotion to his role, Osage language teacher/translator Christopher Cote says the actor was "very serious about" speaking the Osage language and getting it right.

"So we met every day, getting the gestures right and the voice right," Cote adds of De Niro, 80.

Related: Leonardo DiCaprio and Lily Gladstone Star in Epic New Trailer for 'Killers of the Flower Moon'

DiCaprio and Gladstone star in Killers of the Flower Moon as Ernest Burkhart and Mollie Kyle, who strike up an "improbable romance," per an official synopsis.

Based on David Grann's 2017 bestselling book of the same name about a true story, the movie "is an epic western crime saga, where real love crosses paths with unspeakable betrayal."

"Oil brought a fortune to the Osage Nation, who became some of the richest people in the world overnight," the synopsis adds. "The wealth of these Native Americans immediately attracted White interlopers, who manipulated, extorted and stole as much Osage money as they could before resorting to murder."

The cast also includes Jesse Plemons, Tantoo Cardinal, John Lithgow, Brendan Fraser, Cara Jade Myers, JaNae Collins, Jillian Dion, William Belleau, Louis Cancelmi, Tatanka Means, Michael Abbott Jr., Pat Healy, Scott Shepard, Jason Isbell and Sturgill Simpson.

Killers of the Flower Moon is in theaters Oct. 20.

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