There's a moment in the Young Vic's new production of Yerma that illustrates why Billie Piper is so captivating on stage. Arguing with her character's husband, Piper flips suddenly from wild-eyed, irrational fury to brittle grief - and everything shifts on her ultra-expressive face.
In Spanish, 'Yerma' means 'barren'. The play - which runs until September 24 at London's Young Vic - is a loose adaptation (like Clueless to Jane Austen's Emma) of Federico Garcia Lorca's 1934 exploration of childlessness. The action has shifted from rural Spain to modern-day London, with Piper playing one half of a couple struggling with infertility.
In a gruelling 100 uninterrupted minutes the audience watches as Piper's character - known only as 'Her' - transforms from a charming (if self-absorbed) middle-class Londoner in a leather mini to a woman disintegrating psychologically as she undergoes round after round of unsuccessful IVF.
Designer Lizzie Clachan's set is phenomenal. The audience sits on both sides of the stage, with the action occurring in a Perspex tank. We are the outsiders staring into a goldfish bowl, or at a slide under a microscope, or perhaps into an incubator.
But it is Piper's performance that stuns. Critics have struggled to find hyperbolic-enough adjectives to do the 33-year-old justice. Theatre critics have described her outing as "devastatingly good", and "earth-quaking". As a result of the rave reviews, the play is now sold out. Earlier this week, there was a long queue for returns and the cast received a standing ovation.
It shows how successfully Piper has expunged her pop princess past. She was just 15 when she released her debut single, Because We Want To. In her purple velvet suit and white crop-top, this was her Robin Sparkles moment; what strikes you watching it now is how little she has changed physically, as though there must be a wrinkled painting in an attic somewhere. She still holds the UK record for the youngest solo artist to enter the charts at No 1.
Back then, Piper was the UK's answer to Britney Spears: fresh-faced and with an excess of attitude, as though a teen singer had been created simply to annoy the establishment. There was even the early wedding - to Chris Evans, who she married in 2001 aged just 18. "No one was more surprised than me," he said recently of their relationship.
While they were together, she quit music and moved into acting, swiftly winning acclaim for her role as Doctor Who companion Rose Tyler. After finalising her divorce from Evans in 2007, she went on later that year to wed Lewis actor Laurence Fox, from whom she split five months ago.
Lucy Prebble, the Enron playwright who has worked twice with Piper, says she considers her both her best friend and her muse: "It's a weirdly old-fashioned idea to have a muse. But if I do, it's Bill." Prebble describes her as "extraordinary" in Yerma: "The reviews are completely fair ... It's her mainlining of emotion that I think an audience really responds to. It pours out, and there's a catharsis to it."
Piper, who has two sons with Fox, discussed Yerma with the child-free Prebble. "It's so apposite for women in their thirties. I wrote her notes on what it's like not to have kids."
'Her' starts off smug - a confessional journalist who glugs champagne with her even more self-satisfied husband. They are the type who support anti-gentrification movements while being part of the problem. It is testament to Piper's talent that you sympathise with her throughout.
The performance is also a reflection of her dedication. John MacMillan, who plays ex-boyfriend Victor in Yerma, reveals that he and the crew call her Michael Jordan. "Like MJ, she could have the worst flu and a sprained ankle but she'd still go out there and score you 50 points. The part she's playing asks everything of her, and by God does she give it everything, every time... She has led us by example."
MacMillan is full of praise for Piper personally too. "Billie is the most hard-working person you could ever hope to share a stage with," he says. "She's as big-hearted as she is brilliant."
She remains surprisingly normal too. During Yerma's press night party Piper and Prebble sloped off. "We were sitting in the back seat of an Uber with her mum and dad, singing our hearts out to Adele on Magic FM."
Prebble adds: "We've been there for each other through massive life crises and picked each other up off the floor."
She concedes she never expected an ex-Doctor Who star to be her inspiration, "but you can never predict. I think in your thirties something changes and your real love affairs become with women again, a bit like the intense friendships you had as a schoolgirl. Men fade away. I've had male muses. They tend to wander off, or worse, start painting you back."
Prebble and Piper are now working together on a new, secret project: "We've hardly told anyone about it because we want to make it as good as possible and maintain creative control before taking it out."
The pair met in 2007, when Piper was cast as high-class escort Belle in Secret Diary of a Call Girl. Prebble was the show's writer and recalls that Piper was fearless. "What's great about Billie is that she's genuinely interested in the unpretty parts of being human, and of being herself... She wants to work on the things that are broken, that are messed up. Lots of actors say they will, but vanity stops them. Billie's the real deal. She's so perfect she even has flaws that make her even more loveable - and she'll show them to you."
Prebble and Piper weren't particularly close until filming ended. "At the wrap party, she said to me, 'I think we should be real friends'. And I probably fakely hugged her and said, 'Of course.' I remember she looked at me properly and said, 'No really, I want to be your friend'. The sheer vulnerability of it. I am still so impressed by her."
Apart from managing to be on such good terms with ex Chris Evans, Piper's most impressive feat is perhaps winning over an initially sceptical thespian crowd. That, an actor well-known on the London stage tells me, is down to her "fierce ambition". Her theatrical debut came in 2007's Treats at the Garrick, where she met Fox.
Piper is now writing too. "She showed me the early script and it just floored me with its raw honesty, but also her trust in me to share it," says Prebble.
Having spent so much of her life in the spotlight, though, Piper is careful about her privacy now. She hasn't spoken about her divorce from Fox, or about dating Johnny Lloyd, formerly of indie band Tribes.
Piper seems to inspire unfaltering loyalty from friends. "I won't hear a word against her," says Prebble, before correcting herself. "Well, I will, but I'll write them down, turn them into a part for her, and she'll break your heart."