She explained, "I tried to understand the psychology of this kind of girl as much as possible. I didn't want her to be just a parody."
She added, "The accent was hard as it was a particular kind of dialect. I also thought a lot about what her parents must have been like."
The film, directed by Sofia Coppola, is based on a group of "Bling Ring" thieves who stole more than than $3 million worth of goods from the Hollywood homes of celebrities including Paris Hilton, Lindsay Lohan and Orlando Bloom.
Watson watched several episodes featuring the Kardashians as well as episodes of "The Hills" to prepare for her part in the film. But she says it's more than just a heist movie.
"It was how about these kids wanted to pretend for just two hours or so that they were actually Paris Hilton and living that life," she explained.
"The Bling Ring" opened at the Cannes Film Festival on Thursday.