Watson said she watched American TV shows like The Hills, Keeping up With the Kardashians and The World According to Paris to prepare for the role.
"It'd be very easy for Nicki to feel like a parody," Watson said. "Somehow I had to understand and empathize with her and that was really biggest challenge, second to getting the accent down. It's quite a specific dialect."
Since the Harry Potter films concluded in 2011 with the second part of Deathly Hallows, Watson has gravitated to more adult films, including last year's coming-of-age tale The Perks of Being a Wallflower and Darren Aronofsky's Biblical epic Noah, which recently finished shooting. She also makes a cameo as herself in this summer's This is the End, an apocalyptic comedy.
"For Emma, it was, I think, really interesting to see an actress that you've seen in other things really transform into a completely difficult character," Coppola said.
Young actors often treat Cannes as a coming-out-party, a place to introduce themselves on a more artistically-minded stage. Watson shined with enthusiasm for her experience with the improvisation-friendly Coppola.
"I could work in a way that was a lot more loose than I was used to," said Watson. "I'm used to really having to stick to my lines because a lot of people know them by heart."