Kevin Costner admits ‘Horizon’ is hardest thing he’s ever done: 'We never stopped working’

Kevin Costner is no stranger to hard work. 

Speaking to Fox News Digital at the premiere of his new film, "Horizon: An American Saga," in Los Angeles on Monday, the 69-year-old actor opened up about the amount of time that went into creating the project and explained why he's so "proud" of the movement thus far. 

"Well, this is my town. This is a company town. I adore what Hollywood can be and what it has been. To be able to bring a movie and be able to be a part of the history of everyone else who's brought movies to Westwood, to the Chinese (Theatre). This is a ceremony. It's something that we go through. We get to celebrate. I'm really proud of the film that people will see tonight. When you let it go, it's now in the river and people can watch this long after I'm alive."

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Costner further explained why his involvement has been one of the most difficult things he's ever done. 

"You can look at the obvious, of how long it took [to finish]," he said. "I directed ‘Dances With Wolves' in 106 days I think, and ‘Horizon’ is arguably as big as that, and we did it in 52 [days]. The only reason that happened was because of the people that were with me. We just never stopped working."

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In addition to starring in the feature film, Costner also directed "Horizon," based on a script he co-wrote with John Baird. The ensemble cast includes Sienna Miller, Sam Worthington, Luke Wilson, Abbey Lee, Jena Malone, Giovanni Ribisi and more.

Speaking more towards Costner's work ethic, cast member Isabelle Fuhrman said the actor is an "incredible leader."

WATCH: KEVIN COSTNER SAYS ‘HORIZON’ WAS COMPLETED IN 52 DAYS

"I feel incredibly grateful. Kevin is such a visionary," she said. "I've looked up to him for so many years. When he asked me to be part of this, I had a full-body reaction of excitement. Western films were sort of a dream that I never thought I'd get to have a moment in. I don't think it's very often where we see women having such deep, interesting characters. There's always a gun fight. I felt like he was trying to create a movie I hadn't seen before, a world that I hadn't seen before."

"You're not looking at just the history of America," she continued. "America is the main character of this film, and it really is about this promise so many people going out there and hoping to make it and all the trials and tribulations that they face. Being able to tell that story, not just on a big scale, but on an intimate, up-close look with each character just felt like such a challenge. Kevin every day made me want to rise to the occasion and be the best version of myself. He's an incredible leader."

WATCH: ‘HORIZON’ STAR ISABELLE FUHRMAN SAYS KEVIN COSTNER IS AN ‘INCREDIBLE LEADER’

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"He's just one of those guys, Kevin Costner leads by example," Wilson added. "He's a really hard-working guy, totally prepared, but he's also open to there being little moments that he hadn't thought of and trying to capture those. He's a top-notch filmmaker."

Speaking to Fox News Digital ahead of the premiere, the Oscar-winning actor revealed that his 15-year-old son, Hayes, isn't the only Costner kid to show off his acting chops in the film.

"They're all supportive of each other," Costner confessed of his seven children. "My oldest son was in a scene in chapter two, and a small part again." 

He joked, "I just try to find ways to trap them."

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Costner shares sons Hayes,15, and Cayden, 17, and daughter Grace, 14, with ex-wife Christine Baumgartner. The former couple finalized their divorce in February after 18 years of marriage.

He also has three adult children — Annie, Lily and Joe — with ex-wife Cindy Silva, and son Liam, 26, with former partner Bridget Rooney.

While his older son makes his debut in the second part of "Horizon," his son Hayes was only 13 years old when he signed on to star in the first film of the post-Civil War drama which Costner began dreaming up more than 30 years ago.

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"That he did it, that he was good in it, that he listened, that he was worried, that he cared… I'm just so happy that he has that now the rest of his life," Costner said. "I haven't pushed my children into this, and this might be his only dabble in it, but he's memorable. And for me to have him, I'll never forget it."

WATCH: KEVIN COSTNER SAYS HE'S ‘REALLY PROUD’ OF ‘HORIZON’ FILM

After months of speculation, Costner confirmed on social media last week that he wouldn't be returning to season five, part B of "Yellowstone."

"Hi, everyone. I just want to reach out and let you know that after this long year and a half of working on ‘Horizon,’ and doing all the things that's required, and thinking about ‘Yellowstone,' that beloved series that I love and I know you love, I just realized that I'm not going to be able to continue Season 5, or into the future," he said on Instagram. 

"It was something that really changed me. I loved it and I know you loved it. And I just wanted to let you know that I won't be returning. I love the relationship that we've been able to develop, and I'll see you at the movies."

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Through his deal with FOX Nation, Costner is set to release a new project in 2025 for the platform and previously hosted "Yellowstone: One Fifty," a series that explores the origins of the national park.