Kevin Costner spoke with ET about his new film, ‘Horizon: An American Saga.’

Kevin Costner is opening up about his decision to not return to Yellowstone for the second half of the fifth and final season.

Costner spoke with ET’s Cassie DiLaura on Sunday, while promoting his new film, Horizon: An American Saga — Chapter 1, and he opened up about his new project, his hopes for his legacy, and his decision to officially exit Yellowstone ahead of the second part of the fifth and final season.

“Yellowstone was a great moment in my life,” Costner reflected. “I remember reading it, and thinking I wanted to do this. Maybe do one season or just one long movie, and ended up doing five [seasons].”

However, with the other projects on his plate and occupying space in his mind, Costner said he found it necessary to change his focus.

“It was important for me to be able to do other things and, you know, try to make that work,” he said, “[But] I just wasn’t able to make it work.”

After months of speculation, reports of feuding with series co-creator Taylor Sheridan, and negotiations with Paramount, Costner took to Instagram on June 20 to officially confirm he would not be coming back for the last batch of episodes.

“I just wanted 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 that’s required, and thinking about Yellowstone — that beloved series that I love, that I know you love — I just realized that I am not going to be able to continue season 5b, or into the future,” a somber Costner shared in the video he posted last week.

Speaking with ET, Costner reflected on his career, the legacy he hopes to build, and the role Yellowstone plays in that legacy.

“I make movies and I’m a part of things that I think are gonna be really watchable. That’s what I want, to be for people to be surprised,” Costner shared. “And Yellowstone burst onto the scene and it was a surprise, and it was wonderful and it was good… and other characters were jumping off the screen, and that’s what I want to be known for, you know?”

“Horizon, even though it’s [rated] R, I want for people to go home and go, ‘You know what? I want to take the kids to see Horizon, because yes it’s violent, but I want them to understand that’s probably what happened to their great great grandmothers and grandfathers.’”

Horizon: An American Saga — which Costner directed, co-wrote and starred in — is a planned four-part epic Western drama series following a group of people exploring the American West across a years-long span before, during and after the Civil War.

With Chapter 1 hitting theaters this month, Chapter 2 coming in August and Chapters 3 and 4 currently in production, it’s one of the biggest undertakings of Costner’s career, and one that is very close to his heart.

According to the star, production on the final two chapters is “going well,” and there’s more to come.

“We’ve shot about 13 days of it, and we’re coming back in August to finish it off,” Costner said. “I saw [Chapter 2] yesterday, and I’m really happy to be able to say that it’s just as good as 1, and maybe better.”

“But the truth of it is, it gets harder than 1. It’s harder on the people. It doesn’t get easier on the people,” he said, adding that it’s hard on him as well with the sheer amount of press he’s been doing to promote the deeply personal project. “I’m just about whooped.”

“But I will continue, because the movie’s important,” Costner shared. “It’s nice to be able to share it, so if talking about it helps that, I’ll do it.”

As for his Yellowstone departure, in response to Costner’s statement last week, a Paramount Network spokesperson said in a statement over the weekend, “Kevin has been a big part of Yellowstone’s success. While we had hoped that we would continue working with him, unfortunately, we could not find a window that worked for him, all the other talent and our production needs in order to move forward together. We respect that Kevin has prioritized his new film series and we wish him the best.”

Horizon: An American Saga — Chapter 1 hits theaters June 28.