“These little ones need all the love they can get,” Sienna Miller said, completing an interview on the phone while on her way to tape another one, for The Tonight Show, on Thursday evening. “So I’m saying yes to a lot, but it’s worth it.”

In this case, the “little one” is American Woman, an indie movie directed by Jake Scott and released on just over 100 screens last weekend by Roadside Attractions. But for Miller, the film is also unprecedentedly huge—her biggest leading role by a long shot, with no A-list male star (as with American Sniper and Live by Night) or ensemble cast (The Lost City of Z, Foxcatcher) to steal focus. “Everything about it felt immense in some way,” Miller said, remembering when she first read Brad Ingelsby’s script. “It is rare to read something like that without a male costar or some other ingredient that would overshadow it being very performance-based.”

American Woman stars Miller as Debra, a grandmother by the age of 32 whose lifelong habit of bad choices—sleeping with married men, angrily confronting anyone who’s wronged her—is transformed when her teenage daughter (Sky Ferreira) disappears. The film follows Debra over the course of 10 years as she raises her grandson, pursues relationships with bad men (Pat Healy) and better ones (Aaron Paul), and eventually grows into the kind of self-possession and responsibility that seemed unimaginable when the film began.

It’s an expansive, consuming performance that might be star-making were Miller not already so famous—an ’00s tabloid fixture who has become the best part of the prestigious films in which she regularly appears, even the ones that barely give her enough to do. American Woman gives her everything to do, and makes a powerful case that, as critic Amy Nicholson put it recently, Miller is the best actor of her generation.

Before Debra, Miller said, “I’d never been given a part where I could be different women within the same person. I find her very funny in moments and very messy, and I could map out a character that began in a really frenetic, frantic way and then find stillness at the end. It was a lot of work, basically.”

It’s work she’s clearly proud of—but Miller also seemed not quite comfortable with the amount of self-praise that comes with flogging a film in which you play the lead. When describing the emotional availability she tapped into to play Debra, Miller interrupted herself—“I’m not blowing smoke into my performance. Some people will hate it, whatever. I’m not saying this is the second coming by any means.” And when talking about the film’s Los Angeles premiere, she pivoted immediately to the people who joined her in the audience, rather than talking about herself: “It’s not an organic experience for me, watching myself on film. But I had several of my close friends in L.A. who came, so we were all sitting together, and that was sweet to have everybody there. And that was sort of what was more exciting than looking at myself.”

She stopped then, seemingly to correct herself—though there was nothing to correct. “Either way you talk about this, it sounds annoying,” she said. “Just write this the right way, because otherwise I sound like a total tit.”

Miller has more reason than most to be wary of how she’s being portrayed in the press. In 2008 she successfully sued a British photo agency for invading her privacy, and has been candid in the past about how coverage of her personal life affected her career in its early years. Even though, like many actors, she doesn’t read reviews (“unless I’m in a really self-sabotaging mood”), she’s remarkably aware of the media narratives that can shape acting careers. Reflecting on 2014, in which she had supporting roles in best-picture nominees Foxcatcher and American Sniper, she said, “It definitely shifted things. The narrative kind of changed. And then I work for a little bit and it disappears. And then you suddenly have more work coming out, and people are like, Oh, it’s another resurgence, and then I’ll disappear again. You know, it’s a cycle.”

American Woman may or may not be another one of those resurgences. It’s a small movie, but small movies with phenomenal lead-actress performances have a way of sneaking their way into the Oscar race. (Just ask The Wife. Or Still Alice. Or Monster.) What Miller now has, no matter what, is indisputable proof of just how much she can do. And even if she’ll never go so far as to praise herself, she seems to know it, too.

“I don’t think I’ve ever reached a point where I’m like, Ah, this is it. Now I can relax and sit back,” she said. “I don’t think I ever would, irrespective of what success I might have or failure I might have. I think it’s always an endless kind of quest for something. And in these moments where you creatively tap into something that you weren’t even sure you were aware of or had—I suppose that’s the closest you can come to some sense of achievement. And I have that irrespective of how [American Woman] does, you know?”

This article has been updated.

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