There’s been much anticipation surrounding legendary filmmaker Martin Scorsese’s next movie, Killers of the Flower Moon, and for good reason. It’s based on a wildly popular book by David Grann, which recounts a true event in which wealthy members of the Osage nation in 1920s Oklahoma were murdered, sparking a major FBI investigation in the region. That’s fascinating material to mine, and Scorsese has enlisted a top-notch cast that includes Leonardo DiCaprio, Jesse Plemons, Robert De Niro, and Lily Gladstone and is about to start production on the Apple original film in Oklahoma with a hefty budget of $200 million. It’s an expensive, grand Western tackling one of America’s biggest original sins, and screenwriter Eric Roth thinks it has the potential to be something incredibly special.

I spoke with Roth recently in conjunction with the Oscar campaign for David Fincher’s Best Picture-nominated Netflix film Mank, on which Roth is a producer, but I couldn’t help but ask about Killers of the Flower Moon, which the Oscar-winning Forrest Gump and The Insider screenwriter wrote.

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Roth revealed that filming on Killers of the Flower Moon finally begins in Oklahoma in about two weeks, and acknowledged that it’s been a long process of bringing this one to the screen:

“I think this is my fifth year or six year on it. And there were some changes that came about that were interesting about what Leonardo was going to play in it. I think in the long run — we all had our moments of trying to figure out how best to portray things because the story is so impactful — and I think we ended up with exactly the right material and that Marty made the right decisions. I just think he's going to make — and obviously I would say this — but I think [of] all my work, this one could be one of the great movies. I really mean that. I think it has all the ingredients, which I don't want to jinx it, but the story is so important.”

The project was originally set up at Paramount Pictures before moving to Apple, reportedly over concerns that DiCaprio was shifting his role from the “hero” of the piece – FBI agent Tom White – to a more complicated, ambiguous character who has a familial connection to the mastermind of the Osage murders.

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Image via Paramount Pictures

Roth noted that Scorsese’s aim with the $200 million film is to make what is in all likelihood one of the last big budget Westerns, given Hollywood’s shift towards almost exclusively backing known IP with budgets that large:

“I know Marty's trying to make a movie that's probably the last Western that would be made like this, and yet, with this incredible social document underneath it, and the violence and the environment. I think it'll be like nothing we've ever seen, in a way. And so this one is, to me, one for the ages.”

I asked Roth if Flower Moon is a full-blown Western, and he said the answer is both yes and no while ultimately acknowledging that to him, it certainly feels like a Western:

“I mean, people will be in suits and things because it's 1921. It's during the prohibition, but the ethos I think is very Western. And also, I think Western justice, about how they said that you couldn't find 12 white men to convict a white man of killing a native American. You’d have a better chance of having them convicted of kicking a dog. And that's kind of the feeling on that. And then also, you have these incredible people, the Osage family that a character comes and marries into, and who's a villain and who isn't. And then into that comes a kind of heroic guy — Tom White, his name was, who Jesse Plemons is playing — who was in the Texas Rangers, and you couldn't get more Western than that.”

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Image via Netflix

Given the rumors that DiCaprio had switched to a supporting role, I asked Roth if Plemons is now the lead of the film, and he clarified that Plemons and DiCaprio’s parts are about equal screentime – it’s just that one is the “hero” and the other character is more complicated:

“I wouldn't say [Plemons is] the lead. I would say that he was the designated hero. But yeah, I think that's fairer because I think the parts are pretty equal and they were always equal to a certain extent, and Leo's part is very complicated and very interesting. It's a smart part for a smart actor to play. I mean, if Montgomery Clift was alive, I think he might think of playing him.”

The excitement was palpable in Roth’s voice, but he cautioned that given the lengthy production period (filming extends through July), audiences shouldn’t expect to see Killers of the Flower Moon in theaters before 2022, but that’s all to plan. He added that Scorsese “knows this like the back of his hand” because he’s been prepping the project for so long. And the writer and filmmaker appear to have gotten along tremendously well, despite the various changes the project went through on its road to becoming a reality.

“Marty is a wonderful, special human being,” Roth added. “I would work with him come rain or shine.”

Look for much more from my interview with Roth on Collider soon.