This is such a hard question! It depends on how you define 'favourite role' - if it's in terms of character, it's Sam, but if it's in terms of acting range then it's Alice.
Sam: Like everyone has said - such a great character. You can see why it appealed to her - Sam is cute and funny, but she's not just a typical one-dimensional character in romance movies. She's rather complex, actually - she seems so strong on the outside but she lies all the time and seems to be rather insecure (at least for me) on the inside. And she's quirky and odd and really, it would be a blast having her as a friend
Alice: Does it seem to you that the film seemed to be about Alice, even though it portrayed all the characters equally? Like she was at the focus of it all. Seriously, the film was so good, and part of it was how strong all the acting was, and Natalie's in particular. She made Alice her own. She was really just incredible - you know what I mean. I can't describe how good she was.