Sienna Miller in awe of 'phenomenal' Kevin Costner

Sienna Miller has revealed she would “go to the ends of the earth” for Kevin Costner.

The 42-year-old actress plays settler Frances Kittredge in Costner’s new film Horizon: An American Saga - Chapter One and has insisted it was a dream role for her.

“I grew up watching Westerns. I think my idea of cinema was a Western when I was a little girl, and then Dances With Wolves was a huge, huge part of my life,” she told Deadline.

“I’d really go to the ends of the earth for him, I think he’s phenomenal.”

Miller, who promoted the movie at the Cannes Film Festival this weekend, also admitted she was left a little unsure after initially seeing the hefty script for the four-part project.

However, a conversation with Costner soon cleared up any of her doubts.

“I got this call that Kevin wanted to talk to me, and then I got sent four scripts and I thought it must be a series,” she recalled. “There are four of them, but they’re so big, so it didn’t really make sense. And then we had this great conversation.”

Costner has been working on the ambitious project since 1988 and even put some of his own money into it.

The star has no regrets about how much he has invested both financially and emotionally.

“I think in everything we do, we somehow have to stand for ourself. We have to stand for the moments that we think are important,” he said. “We can’t be consumed with making our pile of money bigger as much as our heart full.

“It’s like, tell your story, hope it’s engaging, believe that it’s engaging.”