“God,” she continues, “he was so right.”

Covering the late 70s through the early 90s, season four of The Crown explores the sad saga of Charles and Diana, whose whirlwind courtship and storybook wedding captivated the world but whose fundamental incompatibility and mutual infidelity resulted in a messy split more than a decade later.

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Previously, The Crown has often focused on obscure or forgotten chapters in British history, but many people vividly remember the Charles and Di era – and have fixed opinions about it. Caron admits that moving into a more recent period “feels uncomfortable”. The delicate subject matter made it even more essential to find the right actress to play Diana. “I really hope none of the royal family watches it,” he adds.

An executive producer on the series, Caron also directed three episodes this season, including the stand-out “Fairytale”, which follows Diana during the lonely, disillusioning months before her wedding in July 1981. In a sequence that drives home her youthful naivete, the princess-to-be skates through the halls of Buckingham Palace while listening to Duran Duran.

“There’s a massive arc for Diana in the season. From a young adult to a mother with two small children and a failing marriage to the future king of England. That’s a big space to occupy and Emma fills it,” says Caron.

In season five, following Diana in the last years of her life, the part will be played by Elizabeth Debicki. “Emma effortlessly walked that line between strength and vulnerability,” he adds.

Though Corrin has no living memory of the princess, who died when she was a toddler, she says she still felt her influence. “I grew up very aware of her being generous and empathetic – a people person – and how rare that was. Whenever I heard about her with someone or watch a documentary I remember thinking, ‘Oh, this is the kind of person who I want to be,’ which is now weird,” she says, laughing. “I went a little too far.”

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And she has Camilla to thank for it, in a sense. Producers were casting the role of Charles’ long-time mistress. Though season three hadn’t even begun to film, Morgan had already written a scene from season four in which Camilla takes Diana, her lover’s fiancée, out for an awkward lunch date/power move. Corrin was asked to read for Diana.

“I remember calling my agent and saying, ‘My God, I think they liked me. I think that’s a good sign?’” Corrin says. “She said, ‘Please don’t get your knickers in a twist.’”

Some months later she was called in for a formal audition with Morgan, Caron and casting director Robert Sterne. She dazzled them with a spontaneous karaoke rendition of “All I Ask of You”, a song from Phantom of the Opera that features in one of season four’s more cringeworthy moments.

Caron had a single reservation: He worried that, with her wide blue eyes, Corrin looked too much like Diana. “Normally we don’t go to someone who is almost a lookalike. We find the best actor,” he says. “But she had both, and that felt a little bit unnerving.”

“Diana is constantly in a state of flux between strength and vulnerability,” says Corrin. “A lot of people refer to young Diana as sweet and shy, and yeah, a lot of us are when we’re 18 or whatever. But she also knew what she was about. When she goes to Balmoral” – for a weekend with the royal family that amounts to an audition to be Charles’ wife – “she knows exactly what she’s doing. It’s outrageous.”

Corrin read biographies of Diana sent by the show’s research team, but the most useful source she found was the documentary Diana: In Her Own Words, which uses recordings of Diana’s interviews with biographer Andrew Morton. Corrin has watched it so many times she says it still comes up in her Netflix homepage.

Corrin trained with movement coach Polly Bennett to understand Diana’s distinctive mannerisms – her bashful head tilt, sidelong glances and tall-girl slouch – from the inside out.

With someone as well-known as Diana, Bennett explained in an email, “the challenge was to look at her afresh, as if we had no idea who she was or what she was known for doing, so we [could] imagine why her behaviours manifested themselves the way they did. We worked together to get to the core of what made Diana who she was, rather than proliferate what we think we see.”

Suddenly being trailed by photographers would have been a huge change for an unassuming nursery schoolteacher who had rarely attracted much attention.

“We imagined laser beams sniping through space towards Emma, which immediately gave Emma the reflex of lowering her chin and shrinking her frame to hide,” Bennett wrote. “Suddenly we have an active reason for Diana’s trademark head tilt, which is far more practical to play for an actor than a direction of ‘just tilt your head’ that can leave them stuck and overthinking.”

Together they also spent many hours researching the psychological impact of bulimia and reviewing footage of Diana’s body language to anchor the performance “in truth rather than mimicry,” she says.

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Perfecting Diana’s accent required a bit less imagination. “I’m kind of a bit posh, so it was not a huge leap,” says Corrin, who grew up in Sevenoaks, an affluent commuter town outside London, and attended Cambridge. But she was moved by the way Diana’s tone dropped the end of each sentence, “which makes everything she says sound quite sad”.

As for her own take on this painful chapter from royal history, Corrin is diplomatic.

“I have come to know Diana better than anyone else, so my sympathy will always lie with her,” she says. “But I also have a huge appreciation of Charles, and what they both had to endure in that marriage. I don’t think you can pick a side.”

Spoken like a true queen. Or at least a princess.

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