Leonardo DiCaprio may be the star of Martin Scorsese’s latest film, "Killers of the Flower Moon," but he is most certainly not its hero.

Based on the book of the same name by David Grann, "Killers" tells the true story of the murders of members of the Osage Nation in Oklahoma by White settlers in the 1920s after recently discovered oil makes the Osage people some of the richest in the country.

DiCaprio plays Ernest Burkhart, a man who marries an Osage woman named Molly Kyle, played by Lily Gladstone, and becomes involved in the dark plot to not only murder her family, but poison her as well. 

Originally, the movie hewed closer to the book’s focus on the federal investigation into the deaths, but as Scorsese and his team worked on the story, the focus shifted to the relationship between Ernest and Molly, and the impact on the Osage people.

Lily Gladstone and Leonardo DiCaprio in a scene from Killers of the Flower Moon

Leonardo DiCaprio stars alongside Lily Gladstone in Martin Scorsese's "Killers of the Flower Moon." (Melissa Sue Gordon)

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That also meant a change in casting, as Scorsese explained to WSJ Magazine, suggested by DiCaprio, who was meant to play the investigator but opted instead for the less than upstanding Ernest.

"The complexity of how that relationship unfolded is shown in the movie in a very interesting way, and we played with how much we wanted to show the audience as far as Ernest’s complicity in some of these murders," DiCaprio said in an interview for the film (conducted before the SAG-AFTRA strike).

He continued, "There were many different versions of this script. One of them was exposing everything from the onset. A lot of it was a slow reveal, but I think Scorsese masterfully has this sort of slow burn with understanding the manipulation from his uncle and he gets further and further and deeper and deeper down this well of atrocious acts."

In an interview with British Vogue, DiCaprio also said the first version, "just didn’t feel like it got to the heart of it."

Close up of Leonardo DiCaprio

Leonardo DiCaprio suggested he take the more villainous role of Ernest in "Killers of the Flower Moon." (Laurent KOFFEL/Gamma-Rapho via Getty Images)

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He continued, "We weren’t immersed in the Osage story. There was this tiny, small scene between Molly and Ernest that provoked such emotion in us at the reading, and we just started to penetrate into what that relationship was, because it was so twisted and bizarre and unlike anything I’ve ever experienced before."

In the same interview, his co-star, Gladstone, said, ""The focus would’ve been the FBI, with Mollie and Ernest being part of the supporting storyline, instead of the central one."

Gladstone had previously told Vulture that the rewrite meant the movie "is not a white-savior story. It’s the Osage saying, ‘Do something. Here’s money. Come help us.’"

The switch up in DiCaprio’s casting was not entirely well-received by the higher ups though.

Leonardo DiCaprio posing with Martin Scorsese

Leonardo DiCaprio, left, and Martin Scorsese have worked together six times. Scorsese told WSJ Magazine the studio was uncertain about the switch in casting and script, but it came together in the end. (Samir Hussein/WireImage)

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"The studio said, ‘We backed the other version, we can’t back this one,’" Scorsese told WSJ Magazine.

Apple TV+ ended up coming to the aid of the "Killers" production, partnering with Paramount to release the film in theaters before its debut on the streaming service. 

The change allowed Scorsese to explore "a story that has real evil in it, evil like I've never covered or ever experienced or researched about before in my life," as he told CBS News.

He continued, "The heart of the entire situation is, love, the trust that goes with love, and then this extraordinary betrayal, and still [be] loving. Now, how do we do that?"

Martin Scorsese on the set of "Killers of the Flower Moon" with Liliy Gladstone.

Martin Scorsese on the set of "Killers of the Flower Moon" with Lily Gladstone. (Melinda Sue Gordon)

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The murder plot, driven by greed, ignorance and racism, is put forth by Ernest’s uncle William Hale, played by Scorsese’s other frequent collaborator, Robert De Niro.

De Niro spoke about his role at the Cannes Film Festival earlier this year, saying he did not "understand a lot about him" and why he committed the murders. However, he claimed it came about from the "feeling of entitlement" and "the banality of evil."

Robert De Niro and Leonardo DiCaprio in a scene from Killers of the Flower Moon

Robert De Niro, left, and Leonardo DiCaprio play uncle and nephew in "Killers of the Flower Moon." (Melinda Sue Gordon)

Gladstone, in an interview promoting the film (conducted before the SAG-AFTRA strike), noted that her character, Molly, forms a genuine bond with DiCaprio’s Ernest, despite what turns out to be ulterior motives.

"Through one lens, you see a very organic attraction forming, and through another lens that Molly is pretty unaware of, you see calculated arrangement happening," she said.

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However, lurking behind their relationship is De Niro’s character, who encourages a marriage to an Osage person because, as Gladstone put it, "There’s good money in that."

Leonardo DiCaprio holds hands with Lily Gladstone in a scene from Killers of the Flower Moon

Lily Gladstone said in an interview that her and Leonardo DiCaprio's characters have an "organic attraction," but there were also ulterior motives at work. (Melinda Sue Gordon)

Gladstone, a well-established actress who has appeared in shows like "Reservation Dogs" and "Billions," did admit to feeling intimidated working with DiCaprio, but those fears quickly dissipated.

"The first day that I worked with him, my hands were shaking naturally, because it's Leonardo DiCaprio," she recalled. "But after getting through those initial nerves, I was just sitting across from an incredible present, generous, talented, immensely talented, actor. And the rest of the nerves about it faded away when we were just so focused on if we were doing justice to our characters, so that was incredibly inspiring."

Leonardo DiCaprio, Lily Gladstone and Martin Scorsese pose together

Martin Scorsese hopes he can showcase an "important" part of American history with "Killers of the Flower Moon." (PATRICIA DE MELO MOREIRA/AFP via Getty Images)

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As the film debuts in theaters, Scorsese ultimately hopes that by showcasing the dark truth to an "important" part of American history, he can make a difference.

"I’m 80 now, and I kept thinking, ‘Well, everybody should know that; everybody should know this.’ There are people who don’t even know about World War II," Scorsese told USA Today. "So what do we do, not talk about it? Let’s show it. Let's talk about it. Because it’s happening again; look at what’s happening around the world."