Sarah Gadon on playing the young Princess Elizabeth in 'A Royal Night Out': A Cinderella story in reverse

Canadian actress Sarah Gadon tells Charlotte Cripps how she prepared for her role and why her relationship with David Cronenberg is so important

Sisterly love: Sarah Gadon (left) as Princess Elizabeth with Bel Powley's Princess Margaret

It is 1945 and King George VI (Rupert Everett) and his wife (Emily Watson), the future Queen Elizabeth the Queen Mother, are deciding whether to let their teenage daughters go out to celebrate VE Day. Princess Margaret, aged 14 (Bel Powley), announces that she is "completely cheesed" at being stuck in Buckingham Palace. It is her sister, Princess Elizabeth, aged 19, the future Queen of England (played regally by Sarah Gadon), who finally gets their permission ("Orf you go!").

A wild night unfolds, particularly for Margaret, who gets drunk, and later asks her parents, "By the way, what is a knocking shop?" This is the story, inspired by true events, told in A Royal Night Out, the new film from Julian Jarrold, director of Brideshead Revisited and Becoming Jane.

When I meet rising star Gadon, 28, at a grand London hotel, she couldn't look less like Elizabeth, with her blonde hair and Canadian accent. Her PA has just delivered some freshly squeezed green juices and ginger shots, to boost her immune system. "You never film in one location, so I get run down and tired from flying a lot," says Gadon.

She was discovered by David Cronenberg, now her mentor, when he cast her opposite Michael Fassbender as Emma Jung in 2012's A Dangerous Method. "I had never met him, but he cast me in the part. I flew to Germany, and I met him in full hair and make-up for the camera tests. It was pretty terrifying," says Gadon. He subsequently cast her in Cosmopolis (2012), as Robert Pattinson's wife and Maps to the Stars (2014), as a movie-star ghost, haunting Julianne Moore's character.

The Toronto-based actress trained as a ballet dancer at The National Ballet of Canada from the age of six to nine, before leaving to attend a performing arts school. She later did a degree in Cinema Studies at the University of Toronto. "My parents were academic so there was no way I was going off to Hollywood to become an actress. They supported me when they saw an acting career could sustain me," she says. "I think my degree helps. I speak the same language as directors, so when they reference films, usually they are films I am well-versed in. In terms of research, it has made me quite studied in how I approach a character. Mine is always very methodical in terms of research."

For this latest role, sure to be her breakout, Gadon says: "I watched Brief Encounter in order to get the heightened pitch of Princess Elizabeth's accent just so." She also took etiquette lessons, watched news clips, and read Elizabeth the Queen: The Life of a Modern Monarch by Sally Bedell Smith. "When she described the night they went out on VE night, she says it is a Cinderella story in reverse. I thought that really captured the whole spirit of the film. It wasn't about putting on this dress and becoming a princess. It was becoming a part of the crowd and what that meant to somebody who wanted to feel normal for a night."

It is known that the two princesses escaped from the Palace to mingle with the crowds and go dancing at the Ritz on VE Day, but in the film version, Elizabeth is given a love interest in the form of the working-class Jack (Jack Reynor), who looks after her after she falls off a red London bus. "It was the romance in the film that excited me as well as the relationship between Elizabeth and Margaret, which was so sisterly," says Gadon.

Filming took place in Hull, where the production team recreated Forties London using more than 300 locals as extras. The interior of Buckingham Palace was shot at Chatsworth House in Derbyshire and Belvoir Castle in Leicestershire, while the scenes at The Ritz were shot in the faded grandeur of the Hotel Metropole in Brussels. Scenes in The Mall and Trafalgar Square were the only genuine locations, for which the extras from Hull were shipped in to perform the flag-waving crowd scenes.

Last year, Gadon starred in Belle, as Lady Elizabeth Murray, who grows up with her aristocratic mixed-race cousin, Dido Elizabeth Belle, and she recently wrapped filming Mika Kaurismaki's The Girl King, due out this year, in which she plays the lady-in-waiting to the 16th-century Queen Kristina of Sweden, with whom she has a lesbian romance.

Meanwhile, in The 9th Life of Louis Drax, a thriller, also out later this year, Gadon plays a woman with Munchausen syndrome by proxy who is harming her child to get attention and she is preparing to star opposite James Franco in a TV series for Hulu called 11/22/63, based on the Stephen King novel.

It is still her relationship with Cronenberg that she values most. "It has been a beautiful working relationship. I believe in auteur film-making and I have always been drawn to that. David is a rare and true auteur. He's somebody who can make his films in an uncompromising way and, because he has the freedom to do that, gives his actors the freedom to be creative."

Gadon, who lives in a secluded coach house in Toronto, with her film editor boyfriend, has no plans to leave Canada, but says that it was her roots in the UK that drew her to the part of Elizabeth. "My grandmother was British and in the Women's Auxiliary Air Force during World War Two and that's where she met my grandfather, who was sailing for the Royal Navy. They married during the war and emigrated to Canada afterwards. They were both in Trafalgar Square on VE night, so when we were recreating that scene in Trafalgar Square, it was pretty powerful to know that they had been there at that moment in time."

'A Royal Night' Out is released in cinemas on 15 May