Kevin Costner Has Nothing But Love for ‘Horizon’ Costar Sienna Miller: ‘She Was at the Top of His List’

Kevin Costner has nothing but love and admiration for Sienna Miller, whom he cast in Horizon: An American Saga. A source exclusively breaks down how they’ve been bonding on screen as well as off to Life & Style.

"Sienna has never had a role as juicy as the one she has in Horizon where she gets to play the full range of emotions, from romantic to tragic, and gets to show off her awesome ability to pull off a convincing historical American accent, which is something she has worked hard at for twenty years,” the insider says.

Horizon: An American Saga “chronicles the incredible epic journey of the expansion of the American West, before and after the Civil War,” according to a synopsis.

The synopsis continues, “Between the Native Americans who saw their lands getting colonized and those who were determined to settle there, sometimes at any cost, history is being written. In a flamboyant fresco where multiple destinies intertwine, dreams and hopes face obstacles and cruelty to offer a cinematic spectacle of exceptional scope and emotional depth."

Sienna, 42, plays Frances Kittredge in the film, a pioneer woman who was brought to the settlement by her husband.

“She looks amazing in the movie too, and she's a 100 percent believer in Kevin Costner. If the movie works, and it just might, she's finally on the A-list and will have her pick of roles. Kevin carefully cast this movie with people he's been dying to work with for years, and Sienna was at the top of that list and has the biggest part after Kevin himself.”

However, it wasn’t just Sienna’s beauty that helped her nab the role.

The source adds that “it's not just because she's Kevin's 'type' or sort of looks like Christine [Baumgartner], Kevin's ex-wife. Kevin genuinely sees Sienna as an underrated, important talent and part of the mission of Horizon is taking her to the next level. Everything about this movie is close to Kevin's heart, including and maybe most importantly, Sienna's substantial, serious and emotional performance.”

Both Sienna and Kevin, 60, attended the 2024 Cannes Film Festival as the Field of Dreams star debuted Horizon: An American Saga for the first time. Five of the actor’s seven children accompanied him on the trip to France, while Sienna attended with her boyfriend, Oli Green, and her daughter, Marlowe.

After the premiere of Kevin’s film, the audience gave the Yellowstone alum an 11-minute standing ovation that seemingly brought him to tears. However, Kevin joked during his Tuesday, May 22, appearance on Jimmy Kimmel Live that he wasn’t technically crying.

“I actually started walking my life backwards for a second. I kind of went back to the beginning and wondered, you know, how I even ended up at a place like that,” Kevin told host Jimmy Kimmel. “We all have these dreams where we want to be. And I’ve had my own. And I’ve taken some really big bites out of life. And life has taken some pretty big ones out of me. When they started the clap and they didn’t stop, I didn’t really know what … And I didn’t f--king cry. I didn’t cry. My eyes were full.”