Speaking before the premiere, Watson said: "Harry Potter feels like such a long time ago; so much has happened in the last three or four years, but obviously it's still very present, it's still being played in people's living rooms. I'm not trying to run away from it … but it's that I've had such an amazing three or four years having a chance to transform into new roles and work with new creative people."
Watson, who took a role in 2011 in My Week with Marilyn and will appear in Darren Aronofsky's biblical epic Noah, said she had relished the chance to work in a freer fashion: "I am used to really having to stick to my lines because people know them by heart, so it was lovely just to be able to ad lib or improvise."
Coppola's film is based on the true story of a group of suburban Los Angeles teenagers, some from privileged backgrounds, who stole luxury goods from the houses of the rich and famous out of a desire to possess their wardrobes and lifestyles.
Watson researched her role, she said, by watching reality TV shows such as The Kardashians and The Hills. She said: "It wasn't so much about the stealing, it was more that they wanted to pretend for two hours that they were Paris Hilton. That they were living that lifestyle for real."
Among the thieves' targets was Hilton, whose doorkey they found under her doormat. Between October 2008 and November 2009, they stole more than £2m worth of items, including "a stash of Rolexes" from Orlando Bloom, according to an account by one of the real "bling ring". The teenagers used Google Maps to identify escape and entry points and social media to know when the celebrities were away from home. They also boasted of their new possessions on Facebook.
The real Hilton makes a brief appearance in a film that piles on the ironies: her home was used as a location, so we see her jewels and designer clothing; the Louis XIV-style armchairs heaped with cushions screenprinted with her face; and her "nightclub room" complete with its own poledancing pole. Hilton was expected to attend the Cannes premiere, said Coppola.
Having a film made about their exploits might be regarded as the final victory for the real-life "bling ring" – but according to Coppola, that was far from her intention. "I changed the names of the characters because I didn't want to make them more famous," she said, adding that it was "not a documentary" and that she was "not too concerned with the reactions" of the people on whom the story is based.
Coppola was born into celebrity as the daughter of Francis Ford Coppola; Watson had it thrust upon her as a child. But Watson distinguished herself from the celebrities touched on in the film. "There are celebrities that create a brand and create a business and a whole job, a whole life, out of other people's interest in their lives, and then there are celebrities or people who have a craft or a trade," she said.
"As long as people understand the difference then it's OK. But there's definitely a difference."
Coppola said what had drawn her to the story was that "it could not have happened 10 years ago" and depended on the rise of social media and a celebrity-news cycle. "It's so different from when I was growing up. There's so much information, and a lack of privacy: these kids knew so much about the people that they felt they [really] knew them. They knew what they were having for breakfast."
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