“It’s a perfect wig,” breathes the Netflix publicist solemnly, letting the phrase hang in the air for a moment. We’re high up in the dress circle of the New Wimbledon Theatre. It is January, and dotted throughout the cavernous red-velvet space below us are a director, 40-odd television crew members, and 166 extras in dinner jackets and silk gowns – channelling more 1980s pomp than a mid-Thatcher-era fundraiser. It is exactly the sort of razzmatazz you want from a show rumoured to cost more than £10 million an episode, but beyond the fabulousness, what you really sense is anxiety-level anticipation. We’re all waiting for the wig.

Marilyn Monroe and Alexander the Great aside, there is no rival in the historical blonde stakes to Diana, Princess of Wales, so it took more than a year for The Crown’s producers to settle on the woman who would – who could – carry off her hair’s feathery fulsomeness for series four of the hypnotically soapy and successful royal docudrama. Spanning Diana’s life from age 16 (when she first met Prince Charles, then dating her sister, Lady Sarah Spencer, at a grouse shoot at Althorp) to 28 (when the marriage was in such a desperate state it will make viewers gasp), it was a part so hotly coveted that Ladbrokes was running odds, James Bond-style, about who would land it.

Then, after a strung-out 12 months, that she says were “mad, suspenseful and crazy”, a near unknown 24-year-old actor called Emma Corrin – who lives in a London flat-share, has a long-standing Diana obsession, and owns a cockapoo called (wait for it) Spencer – was offered the once-in-a-generation role, to play alongside Olivia Colman, Helena Bonham Carter and the great and the good of British acting royalty.

Emma Corrin playing Princess Diana in 'The Crown'

Emma Corrin playing Princess Diana in 'The Crown'

The papers duly went mad, before secrecy descended. During the lengthy shoot for the upcoming series (they began filming in that now hard-to-recall summer of 2019), the paparazzi – with unsettling persistence – only managed to get a handful of long-lens snaps of Corrin in full Diana garb.

Back in Wimbledon, a hush descends as the crew’s walkie-talkies crackle, “Emma to set.” By the time Corrin emerges from the wings, I am actually holding my breath. With minimal fanfare, she removes a long puffer jacket, revealing a divine replica of the bias-cut ivory silk dress that Diana wore to dance – with ballet dancer Wayne Sleep – to “Uptown Girl” at the Royal Opera House in 1985, as Prince Charles (now played by actor Josh O’Connor), by then her husband of four years, gazed on curiously from his box.

Emma Corrin and Josh O'Connor on the set of 'The Crown'
Photo: Vera Anderson/WireImage

Emma Corrin and Josh O'Connor on the set of 'The Crown'

Photo: Vera Anderson/WireImage

Watching the stage, it’s surreal to hear Billy Joel’s song start to play. Even more surreal, Corrin’s body suddenly seems to lengthen, her chin drops and her shoulders take on a tiny, telltale hint of nerves – the precise stance you may recall from watching Diana’s wedding day or her minefield walks, all caught in the glare of hundreds of flashbulbs. It gives me serious tingles, a sensation I’m reminded of in June, when Corrin and I meet on Hampstead Heath, and she tells me about the hook words she developed to be able to fall in and out of Diana’s accent on demand. “All right?” she says simply, titling her head and gifting the two syllables every last ounce of Lady Di’s Sloane Ranger-got-cool realness. It is like seeing a ghost.

I recall thinking, both in Hampstead and in Wimbledon, “I hope she’s ready for what’s coming.” Because, thanks to Netflix’s 193 million-and-growing subscribers, everyone is soon going to be talking about this performance. And thanks to the “Diana Factor” they are going to be talking about the series like never before, too, with Corrin set to play the whole nine yards of the years chronicled by Andrew Morton. Her knockout debut at Balmoral, early marriage, royal tours and motherhood are all on the table – but so, too, are infidelity, bulimia, and the frequently painful psychological extremes of royal life. The script affords Diana a great deal of sympathy – so much so you can almost hear a fresh round of tabloid headlines rumbling into view.

The actor set to play her – who hails from Kent via Cambridge University, where she was a star of the student drama scene and picked up an agent – is not a brash fame hunter. We hung out at Vogue’s BAFTA party earlier this year; she was watchful and hesitant amid the hustle of it all, dressed in a Celine men’s suit. She’d still been at university until relatively recently, and one wondered if all the looming attention might feel a bit much? But apparently there’s a rod of ambition in play.

Emma Corrin arriving at Paris Fashion Week's Miu Miu AW20 show
Photo: Jacopo Raule/Getty Images

Emma Corrin arriving at Paris Fashion Week's Miu Miu AW20 show

Photo: Jacopo Raule/Getty Images

“I’m excited for people to see it,” she says on the heath, many weeks – and a lockdown – after she wrapped filming. It’s one of those slightly drizzly, swampy London days, but we decide to go for a stroll, Spencer in tow. Un-Diana’d it is hard to spot the factors that led to her chameleonic transformation on screen. Her neat brown hair framing a face of pure symmetry, wearing jeans and a Marc Jacobs “I Hate Art” sweater, she is so neutral-looking that you imagine she could play anyone. “Some people are very weird about her,” she says, thoughtfully of Diana. “I just have to realise that this is someone who is so universally adored, everyone is going to have an opinion.

She’s certainly done the graft. Corrin, the daughter of a businessman and a speech therapist, had just left university and had a job packaging “period-proof lingerie” for an online shop when the initial call came that would change her life. Her agent told her that Nina Gold – The Crown’s booker and renowned casting director – had been in touch to say that even though they weren’t auditioning Dianas yet, would Emma be willing to come in and help out by reading with O’Connor and the five potential Camillas for series three. “She was like, ‘I know you’re going to get excited. Don’t. They just need someone to read with them.’”

Halfway through the day, the director asked if she wanted to “just work on some Diana stuff”, and they put her on tape. It was the summer of 2018, nine months before a decision would be made known, and yet, an obsession took hold. On the one hand, she says, “It was like: ‘One day I want to go to the moon – that would be fun.’” But she felt a deeper connection to the role, too.

Despite being only a year old when Diana died, Corrin had long been drawn to the legend of the People’s Princess and was fascinated by old photos of her – “There was a joke at school because my mum looks a lot like her.” In fact, her mother helped her when the time came for her formal audition. “My mum and I used to sit in cafés, run lines and do the voice.” It’s not an easy one to master, she explains. “She’s Sloaney but not just posh. She has a sadness to her voice that’s quite authentic.”

Once she’d watched the documentary Diana: In Her Own Words “about a hundred times”, she was ready. The day came to meet writer and creator Peter Morgan and all the bigwigs. “Honestly, I think it was one of the best two hours of my life,” she says. She morphed into Diana as gangly teen, Diana at a brittle lunch with Camilla Parker Bowles, Diana as emerging megastar. She promised them she could dance, and at one point had to sing along to a backing track of “All I Ask of You” from Phantom of the Opera. Soon, she was summoned to “some mad manor house” for a chemistry read with O’Connor, but within moments of arriving, the costume department were taking her measurements and everyone was beaming at her. They offered her the role right there on set.

It was March 2019, and her career had started buzzing, with parts in 1970s-set Miss World dramedy Misbehaviour with Keira Knightley, and the crime series Pennyworth. But Netflix wanted to wait to reveal its Diana. “I didn’t tell anyone for a while,” she says. “I love my mates but I think it would have got out.” Apparently she was behaving so battily, they eventually guessed, and later, “my friends from school did this incredible thing, where they made me a scrapbook filled with all of the screenshots from our group WhatsApp, where I had said, ‘Oh my God, guys, I’ve been invited to read.’ Or a random conversation we’d had four years ago when I said, ‘Isn’t Diana amazing!’”

But then, of course, she had to actually play her: one of the most famous women of the 20th century, still passionately present in many minds and making headlines. “She has the most enduring celebrity presence,” says Corrin, as we meander across the heath towards her flat for a cup of tea. “An icon, in a way. She had this incredible mesh, this synergy of extraordinary grace and happiness and joy, but also a huge underlying mystery of sadness. I think that’s why people were drawn to her. She gave a lot but also you could sense that maybe there was something else there. Something else that I couldn’t quite put my finger on.”

Pre-filming, the movement coach Polly Bennett visited Corrin’s flat. “I remember us just lying on the floor and asking, ‘Who is she? Let’s find an animal to get us hooked on to her personality and her movement.’” At first, Emma thought Diana was a deer – “a deer in the headlights” – but Polly felt that was a little obvious, and pointed out that Diana wasn’t scared all the time. It took a while for it to click. “But I suddenly said, ‘Polly, she’s a cat!’ I ended up watching a lot of cat behaviour videos.”

“I feel I’ve got to know Diana like you would a friend,” she continues. “I know that sounds really weird, but I get a great sense of companionship from her. I suppose, over time, you kind of start to patch together a sense of empathy and a sense of understanding. I love figuring people out.” Is it true you spoke to her therapist? “I didn’t speak to her therapist. I spoke to her private secretary, Patrick Jephson, who said that she was so funny and so happy so much of the time – I loved that.”

Emma Corrin arriving at the Excelsior during the 77th Venice Film Festival in Venice, Italy
Photo: Jacopo Raule/Getty Images

Emma Corrin arriving at the Excelsior during the 77th Venice Film Festival in Venice, Italy

Photo: Jacopo Raule/Getty Images

Like Diana, Corrin has now had the Vogue cover treatment, with the common thread being the princess’s favourite hair genius, Sam McKnight. “I was like, ‘These hands have touched Diana’s hair,’” she marvels of the day. Naturally, they got chatting. “She got him the filthiest birthday cards ever. And he would ask, ‘Di, where did you get these?’ And she’d say, ‘Oh, I have some people.’ I just love that. Her sense of childishness. I really relate to that.”

More often though, says Corrin, the filming process was impossibly moving. Capturing the toxic she-said-he-said marital fractures took its toll, as of course, did the bulimia scenes. “It was very difficult,” she says of doing her best to treat the issue with care, especially for more vulnerable viewers. “You have to go to a really dark place, understanding that there’s a lot of management and routine that goes into it. It becomes quite ritualistic.” Then there was the wedding morning. “It was incredible, because of the significance of what I was wearing. But the effect it had on everyone in the room was quite terrifying,” she says. “We were filming the scene when you first see her in the wedding dress – I think it was Lancaster House in London – and I had a team of about 10 people helping me put it on, because it’s massive. I walked out and everyone went completely silent. More than anything else I wear in the series, it’s so… It’s her.”

“There was a reverence,” she goes on. “I hadn’t felt strange before. We’d done lots of intense scenes but this was the first time where it was, ‘Woah.’ It was like a presence.” It’s one she has now left behind. As is The Crown’s way, the part has been recast for the next series (Elizabeth Debicki will be taking up the wig). Having arrived at Emma’s place – mansion block, flatmates out – she makes tea and we sit on the balcony off her bedroom. It’s that rare moment when you catch a person in the last days of an old life, just before they’re snatched by fame. Everything about her conversation feels on the cusp. She talks about the first time a photographer followed her and her ex-boyfriend through Soho; of how work pressure meant the relationship didn’t survive lockdown; of the low-level rumble of her anxiety; of how she’s getting advice from her new friend and mentor Helena Bonham Carter, whose house she had spent the previous day cackling in; of how excited she is to be working with the stylist Harry Lambert; of all the amazing fashion she’s going to wear, all the directors she’s Zooming, all the messages, all the dazzle, all the promise.

The parallels have not escaped her. The paparazzi swarmed The Crown’s sets, desperate for shots of her, just as she was attempting to channel a woman going through the same tricky adjustment. She appreciates hers is on a much smaller scale, but says: “It was very strange.” She quietly gazes out across the rooftops of north London. So are you nervous for what’s coming, I ask? She nods. “But I’m excited as well. I’m really excited.”

Series four of The Crown will be released on Netflix on 15 November.