Watching Leonardo DiCaprio, Lily Gladstone Left “Killers of the Flower Moon” Author 'Rattled and Shaking' (Exclusive)

Author David Grann on what it's like seeing his book become a Martin Scorsese film starring Leonardo DiCaprio, Robert De Niro and Lily Gladstone

Watching the likes of Martin Scorsese, Robert De Niro, Leonardo DiCaprio and Lily Gladstone work their magic, especially in a film as richly painted as Killers of the Flower Moon, is a singular experience. Just ask the man who wrote the book.

"As a person who spends much of their time in archives looking at old documents, the last thing you ever expect is that a movie will be made," author David Grann tells PEOPLE.

In the film, Gladstone plays the real-life Mollie Kyle, whose family and community in the Osage Nation of 1920s Oklahoma were the targets of serial killings. DiCaprio plays Ernest Burkhart and De Niro, William King Hale.

"I really embarked on this project because it was a part of American history that was deeply known to members of the Osage nation, of course, but was largely excised from most Americans’ consciousness," Grann says. "It's bringing attention to this history and, hopefully, filling in some of those blank spaces in our memories that for too long had been there."

Related: Killers of the Flower Moon True Story: All About the Real Events That Inspired the Martin Scorsese Film

Grann's book, Killers of the Flower Moon: The Osage Murders and the Birth of the FBI, came out in 2017. Taking it from the page to the screen gave the writer a front-row seat to how some of the greats do their work, which he could only describe as "surreal." After spending years bringing to the page the "dusty records" of the real people the movie follows, watching the characters become fully embodied on-screen felt remarkable, Grann says.

"To see the dedication Leonardo Dicaprio brought to bringing Ernest Burkhart to life was really powerful, and I hope it brings more attention to this history into this culture," he says. Grann was impressed by the actors' and production team's dedication not only to their roles, but to the Osage community.

"The film and the book are two very different mediums," he explains. "And so I think the the film and the book hopefully are arriving at the same kind of deeper truth about this story, about these crimes, about the nature of complicity, about what happens when people are motivated by greed, but also don't look upon another people as individuals with souls, and so commit these systematic, murderous crimes."

With subject matter as heavy as Killers of the Flower Moon, it was especially important to Grann that the Osage people be represented accurately, and the film undertaken with the sensitivity and care the subject deserves. He said that stepping onto set and seeing Osage people he knew from researching the book put his mind at ease.

Related: Granddaughter Recalls Killers of the Flower Moon Villain Played by Leonardo DiCaprio: 'Took Away My Family'

"I think that authenticity, that involvement, that commitment on the part of the movie-makers on the whole production team to make sure that this was done in conjunction with the Osage was really important," he says.

That dedication extends to the biggest names on set, with Grann calling DiCaprio's process and performance "not just magical" but a testament to his dedication to his craft.

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"I got to see the actors really research their roles and really spend time in the community to develop and to bring these people to life," Grann says. "When I watched the film, I was just almost shaking, rattled at moments and also overwhelmed by emotion. To see these people being played, and being played so well, really brought it to life."

Gladstone made history when she won a Golden Globe for best actress for the film, and the people who embody that history agreed that she did it justice. One of Mollie's descendants told Grann that "nobody else could have played Mollie," the author says.

"When you see that connection, when you see that response you know they got it right," he adds.

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