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Sienna Miller is having the 'best' time at the ‘liberating’ age of 40

The "Anatomy of a Scandal" star told Sunday TODAY's Willie Geist why her 30s felt "claustrophobic" and "blinkered."

Sienna Miller has entered a new chapter in her life.

On December 28, the "American Sniper" star turned 40 and she tells Willie Geist on his "Sunday Sitdown" podcast that she's been having "the best" time ever since the milestone birthday.

"I wouldn’t change it for anything," she told Geist. "I don’t think I’ve ever felt better. I don’t. I just turned 40 ... (and) everyone says, you know, 'Life begins at 40,' but that felt like a kind of platitude for the fact that you’re getting over the hill. I actually have never felt more settled, content."

On a recent walk through New York City, Sienna Miller tells Willie Geist that she feels liberated at age 40.
On a recent walk through New York City, Sienna Miller tells Willie Geist that she feels liberated at age 40.Nathan Congleton / NBC

Miller explained that being 40 felt both "liberating" and "nihilistic"' because she stopped caring about what other people thought of her, especially those who never met her in real life.

"(There's) nothing you can do about anyone else’s opinions," she laughed. "I mean, I think, as a young woman, I was really aware of what people thought, and I was sort of dissociated from the persona that the media were creating, which was selling papers."

"But it just felt like I was a serious person, in my mind, frivolous and, like, chaotic, for sure, but not this sort of fluffy, light person," Miller explained, "and it was - I really felt the need to prove myself. Now I'm like, whatever. Which is so nice."

During her career, Miller has been nominated for two BAFTA Awards and one Critics Choice Television Award.

Her new role as Sophie Whitehouse in Netflix's "Anatomy of a Scandal" has also pushed her acting to new heights as she plays the wife of James, a British politician who is accused of sexual assault.

"I understood the world she was from," Miller said of her character. "I liked how acerbic and contained she is."

"I’ve been in sort of somewhat similar situations of, you know, knowing that a story’s gonna break, and the horror of that feeling, and the media, and that’s definitely part of my past," she said.

Although Miller didn't want to give too much away about the show, she did say that there are "many twists" that happen in the series and she's very proud of how the writers developed her character.

"I also like that she begins as someone very, very different to who she ends up being. And so that arc was good," Miller said.

However, Miller does want to take on some different projects in her career. She says that doing a play "every two years" is on her list along with directing and producing more projects.

Sienna Miller as Sophie Whitehouse in season 1, episode 3 of "Anatomy of a Scandal."
Sienna Miller as Sophie Whitehouse in season 1, episode 3 of "Anatomy of a Scandal."Ana Cristina Blumenkron / Netflix

“There are so many people I want to work with," she said.

Right now, Miller is filming her new project, "Raised Eyebrows," a drama about the life and times of Groucho Max, which is based on the book, “Raised Eyebrows: My Years Inside Groucho’s House.” And she can't help but think about how much her life has changed from her 30s.

"I think the 30s felt quite blinkered, and I don’t know, quite claustrophobic," she said. "And suddenly I’m, like, 'Oh, there’s a whole other, to get into metaphor, in our field. Or expansive space that I am excited to fill with all sorts of things."