Wearing a black peplum dress and wide-legged trousers by Dior Couture, she said before a gala screening at London's Mayfair hotel: "It was challenging. I was very nervous.
"Having played someone who was so restrained and very English to playing this free-spirited American girl, which was at the other end of the spectrum for me, it took a while to break down a lot of the barriers I didn't even realise I had put up for myself.
"So when I did, it felt very liberating. I'm very proud of the film."
Emma revealed she would be open to returning to the world of Harry Potter.
Author JK Rowling has teased that there could be more from the wizarding series after revealing she has left "the door ajar" for more books in the future.
"Of course. I'm open to anything," said the actress.
Emma also said she can't wait to read Rowling's first adult novel, The Casual Vacancy, denying she has had a sneak preview.
Actors Logan Lerman and Ezra Miller also star in The Perks Of Being A Wallflower, which opens in cinemas next Wednesday.