Chapter one of 'Horizon: An American Saga' hits theaters on June 28.
Horizon: An American Saga almost had another name added to its star-studded cast list.
ET's Cassie DiLaura sat down with Horizon: An American Saga director and star Kevin Costner as well as stars Sienna Miller, Sam Worthington, Abbey Lee, Luke Wilson, Ella Hunt, Isabelle Fuhrman and Jamie Campbell Bower ahead of the Western film's release. That's when Miller, 42, revealed that her 11-year-old daughter, Marlowe, was almost an extra on set.
"My daughter came," Miller began before admitting, "She was gonna be an extra but she chickened out."
As to why Marlowe got stage fright, Miller isn't sure. But she thinks it would have been a great environment for kids to work on thanks to Costner.
"I don't know," she said. "She just got shy but it was a perfect set for kids to be on because there were animals everywhere and people in incredible costumes, and he's such a family guy. I have this photograph of Marlowe and Kevin around the monitor and he's got one headphone in his ear and he gave her the other, he's got his arm 'round her and I have a picture from the back and it was so special. He's such a family guy."
Miller said her daughter -- whom she shares with her ex, Tom Sturridge, whom she was with from 2011 to 2015 -- is more likely to direct than act in the future.
"She says she doesn't," Miller said of Marlowe having the acting bug. "I think she's a bit of a director."
She continued, "She makes films and they're kind of amazing."
Meanwhile, Worthington said his kids "would've burned that set to the ground."
The mother-daughter duo dressed up for the premiere of Horizon: An American Saga in May at the Cannes Film Festival. Marlowe made her red carpet debut wearing a satin white mini dress with an oversized pastel pink bow detail next to her mom in a sheer periwinkle blue gown by Chloé. The two were there with Miller's boyfriend, Oli Green, who opted for a classic black suit.
Marlowe may not appear in the film, but Costner's 15-year-old son, Hayes, is making his acting debut in his dad's new movie playing Miller's son.
"Hayes, he's so sweet," Miller told ET. "He was so excited to be there, he was so serious about the work. Kevin really held him to task, he didn't give him special treatment, he was hard on him and I think Hayes loved it. He's wonderful and I think would do more and more of it."
ET spoke with Costner in April about co-starring with his son and how watching Hayes grow up served as a constant reminder that he had to get moving on Horizon: An American Saga.
"I named him after the character I play in this. My character's name is Hayes Ellison," Costner said. "And I named Hayes that character. So I was watching him grow up and I was thinking, 'Man, I better get with it and make this movie.'"
Hayes -- who is the second youngest of Costner's seven children, three of whom he shares with recent ex-wife Christine Baumgartner -- did not have to audition for the role.
"No, listen. I don't give great parts away. I understand that there's young actors out [there] that would kill for these parts," Costner explained. "And so mine have to respect the profession. But in certain instances, this was something I thought he could handle. I wanted him close and so I did it. So we're locked together in a film forever. Actually, I'm not in any scenes with him, but I directed him when he was little and now he's big."
He added, "I captured him and I brought him on, and he's very good in this small part that he's done. If you see it, I think he has an impact on the movie."
Chapter one of Horizon: An American Saga hits theaters on June 28, followed by chapter two on Aug. 16.
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