With "American Woman," Miller finally got the chance to step into the spotlight. Here, she explains why it took so long.

Jake Scott’s heartbreaking drama “American Woman,” about a woman who raises her young grandson after her daughter goes missing, is a smart and progressive film rooted in the female experience. More than that, it stars Sienna Miller as the eponymous American woman, marking the first lead role for the actress in a 20-year career.

Miller is best known to American audiences for often playing secondary “wife” roles in films like “The Lost City of Z” and “American Sniper,” and while she’s steadily worked on the big screen for the last two decades, she’s never been given the chance to take on such a substantial role. While the theater world has offered better opportunities – with major turns in “Cabaret” and “Cat on a Hot Tin Roof” – movies have often cast her as a pretty face or a reliable female shoulder to cry on.

“I’ve never carried a film,” Miller said in a recent interview with IndieWire. “To be in every scene was really daunting and really challenging, without having a bigger male costar to hide behind and blame things on or being a vehicle for someone else’s film.”

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Scott’s film follows Miller as Debra, a blue collar single mom still making the same mistakes she made as a teenager (mostly involving terrible taste in men) whose entire life is upended when her beloved daughter (Sky Ferreira) vanishes. Like her mom, Ferreira’s Bridget had a baby when she was just a teen, leaving Deb to raise him as as she cycles through a decade’s worth of grief, shame, and growth over the course of the film.

For Miller, the chance to take on a character who was “really battered by life” but who ultimately becomes “somebody who gets back up and carries on and tries again” was too good to pass up. “This kind of indomitable spirit, the moxie of this character, it was a dream role,” she said.

Miller has dabbled in the blockbuster milieu, including a bit part in “G.I. Joe: The Rise of Cobra,” but she’s mostly spent her career playing supporting roles in smaller features. “American Woman” was the chance to finally correct that trajectory. “It felt epic to study a woman and fall in love with this woman and to get to play her from a young age,” she said. “The arc of it was so rare. I just haven’t read anything like that. I’ve never been offered a part this good.”

The actress didn’t balk when asked about the film being written and directed by men — Scott directed off Brad Ingelsby’s screenplay — despite being rooted so deeply in the female experience.

“I know many women who would be capable of telling a man’s story,” Miller said. “So Brad Ingelsby, a straight white guy, writes this film really about women and it’s the truest thing I’ve ever read in terms of representing what that’s actually like. And Jake is just one of those men who I think respects women more than he does men, and it would take that to do this right.”

Making a movie in which her character and arc is the focal point came at a prescient time for Miller, not just on a personal level, but as it applies to the ever-changing post-#MeToo industry. The actress has already seen some of those changes take root for her, including better pay.

While she declined to name the project, Miller said she has just come off a job in which she was finally paid a rate commensurate with the men in the cast. “It’s amazing that the disparity between the gender pay gap is closing,” she said. “I think it still has a long way to go, but you understand how being undervalued and underpaid for the job that you do, it’s kind of essential to how you view yourself and value yourself. That was a really significant moment, because I understood how it must’ve felt to men all these years. It made me braver in my ability and in the choices that I made.” Her upcoming projects include “The Loudest Voice in the Room,” in which she plays the late Fox News scion Roger Ailes’ wife, and the cop drama “21 Bridges.”

The progress has made her reconsider what she settled for over the years. “I look back on what I accepted, what I was genuinely grateful for, and it’s absurd to have kept women down in that way for that long,” Miller said. “I just feel increasingly as I get older, there is more light on the the differences and how gendered I felt or how gendered people’s behavior has been to me, I feel like I’ve got a long way to go.”

Roadside Attractions and Vertical Entertainment will release “American Woman” in theaters on Friday, June 14.

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