To cut or not to cut a fringe? Without being too superficial about it, you can’t play the Duchess of Cambridge – Kate Middleton as was –without taking her lustrous locks into account, even if you aren’t attempting an impersonation.

“It’s amazing really to make the front cover of most newspapers just for the very fact that you’ve had a hair cut. When her fringe was cut (in September) suddenly that was massive news,” says Jennifer Bryden who plays Kate in King Charles III, the phenomenally popular, award-winning play by British playwright Mike Bartlett.

In fact, Bryden won’t be sporting bangs when the British production arrives at Sydney Theatre Company later this month.

“I wanted to. I angled for it but because we weren’t doing impersonations they felt that actually having long dark hair was enough. So I’ve kept mine long and we put it in hot rollers,” she says.

Ben Righton who plays William has a much healthier head of hair than the Duke of Cambridge with his receding thatch.

“He was slightly dreading the fact that they were going to suggest shaving a bald patch,” says Bryden with a laugh.

King Charles III premiered at London’s 325-seat Almeida Theatre in 2014. Directed by Rupert Goold, it quickly became the hottest ticket in town and transferred to the West End. When a Broadway season was confirmed, a second company was formed to take the play on a UK tour. Led by Robert Powell as Charles, that company is en route to Sydney.

Described as “a future history play”, Queen Elizabeth II is dead and Charles finally ascends the throne, Camilla at his side. But when he refuses to sign a bill restricting the freedom of the press, he triggers a constitutional storm. With civil war brewing, there are suddenly tanks outside Buckingham Palace.

Praised by New York critics as “flat-out brilliant” and “breathtakingly audacious”, Bartlett’s Buckingham Palace drama about a monarchy in crisis is written in iambic pentameter, giving the play a Shakespearean feel infused with a dash of Fleet Street.

Righton says he was “blown away” when he read the script. “That kind of writing does a lot of the work for you because it tells you which words to stress. It forces you into a style of talking. What is brilliant about Mike’s script is it refers to all kinds of modern things but in verse. I love it. It pushes the play along at a wonderful pace,” he says.

Will and Kate are among the most photographed people on earth but both Righton and Bryden decided to focus on the script itself rather than taking a forensic look at the young Royals when preparing for their audition.

“That turned out to be the right decision. Something we were told early on in rehearsals is that this wasn’t about impersonating. We were to approach the text as we would any text and approach the character as we would any other character. And only then, at the end, were we allowed to add suggestions of mannerisms that we had observed in real life,” says Bryden.

“They were never after imitations,” agrees Righton. “It’s like an alternative reality this play, a ‘what if’ Charles were to take the throne.”

When it came to observing mannerisms, Righton noted that the Duke of Cambridge speaks in a “breathy” way, fiddles with his hands a lot and often has them in his pockets. “He’s left-handed. I’m right-handed so I’ve had to teach myself to be left-handed,” says the actor.

“If you look at Will for any length of time he’s – I’m trying to find a nice way of putting this – but he’s quite bland. He’s very straightforward and polite and he smiles a lot so there’s not a lot to go on. He’s a blank canvas.

“Because he’s been photographed since he was born you feel you know so much about him but you realise that we don’t. They’ve done a very good job of keeping his life private and what makes the bloke tick is very hard to find out anywhere. I know two people who went to school with him and I tried to get them to tell me a bit about him but they were very loyal and haven’t said a word. I can’t get anything out of them,” says Righton.

Bryden believes that the way Kate dresses has a strong influence over the way we perceive her. “I am lucky with the silhouettes of the costumes she wears and the heels. Once I was in costume that really helped.

“It’s amazing what the power of the imagination does,” adds Bryden. “In real life I don’t think any of us look particularly like the members of the Royal Family. There are similar shapes, colourings and heights but that’s about it. But actually once we’ve got the costumes on and the characters are introduced in the first scene, the power of the imagination lies with the audience to see the person they are used to seeing in the public eye.”

Beneath the fashionable outfits and flowing hair, there’s a backbone to Bartlett’s Kate that comes as a surprise. Portrayed as a shrewd political operator, who supports and motivates William, she has been compared to Lady Macbeth.

“She definitely wears the royal trousers…. In our play she’s a very commanding figure,” says Righton with a chuckle.

Careful not to give too much away, Bryden says: “She is the key operator, the person who makes the changes in the action of the play. She’s the one person within the Royal Family who can look at what’s going on objectively because everyone else is too tied up with their own family drama. Because Kate is new to all this, she is the one with the outside eye.”

Before she had any idea that she would be auditioning for the play, Bryden saw King Charles III in the West End from a seat in the gods with a friend of hers.

“It was so fascinating in the interval hearing all the discussions. So often everyone just makes and a beeline for the bar and it’s about what they’ve been doing that day. Here, everyone was talking (about the play), whether it was politics or family or actors playing real people or the monarchy. It was amazing. My friend said, ‘you should play that part one day,'” says Bryden.

None of the Royal Family has been to see the play. However, Tim Piggott-Smith who played Charles in the original production, received a letter from a member of the staff at Clarence House, Charles’s official London residence, pointing out that Charles doesn’t wear a wedding ring.

“I think everyone has taken that as a bit of a nod that they’re watching and hopefully approve,” says Bryden.“I’m sure they know all about it but I don’t think they would ever come and see it.”

King Charles III, Roslyn Packer Theatre, March 31 – April 30. Bookings: www.sydneytheatre.com.au or 02 9250 1777

A version of this story ran in the Sunday Telegraph on March 13