Kevin Costner Reflects on 30 Years Since 'Beautiful' “Wyatt Earp” as He Premieres New Western “Horizon ”(Exclusive)

The actor and filmmaker looks back on 'Wyatt Earp' amid its 30th anniversary and the newest project in his Western arsenal, 'Horizon: An American Saga'

Kevin Costner is still exploring the West, three decades after Wyatt Earp.

Directed by Lawrence Kasdan and co-written by Kasdan and Dan Gordon, the epic biographical Western turned 30 on June 24 — just days before Costner's Horizon: An American Saga — Chapter 1 finally hit theaters 35 years after the Oscar winner, 69, first had the idea for it.

Speaking with PEOPLE, Costner says, "I loved making Wyatt Earp with Lawrence Kasdan and our cinematographer, Owen Roizman. It was beautiful."

The film followed Costner's titular Earp, based on the real-life lawman and gambler in the American West, and featured a star-studded supporting cast of Dennis Quaid, Gene Hackman, Michael Madsen, Catherine O'Hara, Bill Pullman and more.

"That movie took about 113 days to make. I loved living in that era," Costner says of the 1994 epic, adding that, in contrast, "Horizon took 52 days, and so I just had to use every trick in the book I know to do a movie as big as Wyatt Earp in half the time."

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Related: Kevin Costner Reflects on His 'Unusual,' Mostly 'Beautiful Life' and Iconic Career: 'I Listen to My Heart' (Exclusive)

Costner both leads and stars in Horizon, serving as the movie's director and co-writer among an ensemble cast that includes Sienna Miller, Sam Worthington, Abbey Lee, Michael Rooker, Jamie Campbell Bower, Ella Hunt, Isabelle Fuhrman, Jena Malone, Luke Wilson and more, as well as Costner's own 15-year-old son Hayes in his debut movie role.

Asked by PEOPLE what it was like having Miller, 42, play Hayes' onscreen mom in the film, Costner says he "couldn't have had a better person" in the role.

"She's our leading lady. She's incredible in the movie," he says. "And she came to me one time, she says, 'You're kind of hard on Hayes.' And I said, 'I know. I need him to be really good, and he's inexperienced and I'm going to help him all the way through.' "

"And in the end, Hayes started doing some things on his own that were just magical, and I'm really proud of his performance," the Dances with Wolves actor explains.

He adds of Miller, "She was just there for [Hayes] professionally. She doesn't overstep bounds. ... She talked to him like an equal, even though he was a beginner. She was just great."

Related: Kevin Costner Tears Up After Hearing Son Hayes' Sweet Words About Working on Movie with His Dad: 'Feels Good'

Looking back at first beginning to develop the idea for Horizon over 35 years ago, Costner tells PEOPLE it took him between a year and a half and two years "to get the script right" for the film, which will span four chapters total.

After initially wanting to make the movie in 2003 and being "disappointed" that he "wasn't able to," about six years later, he and co-writer Jon Baird realized Horizon would need to be in multiple parts — which gave Costner pause at first, before he dived in and made it a reality.

"You can't get one made, so what in the world makes you think that anybody's going to be interested in four?" he says. "And nobody was rushing at it, but I loved it even more."

"So I just don't fall out of love that way," Costner adds. "I just kept loving it more and thinking, 'No, this is what I want to see. I wish somebody would've made this movie for me,' and so I turn around and make it for other people."

Horizon: An American Saga — Chapter 1 is in theaters now, to be followed by Chapter 2 on Aug. 16.

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