Yes, I think Kristen Stewart has played the role of 'Bella Swan/Cullen' quite well. She's delivered her dialogues well, acted well and I could relate to her since I read the کتابیں first. An actor plays the role well when he/she gets into the shoes of the character and I feel that Kristen has been great in playing the role of a good daughter, a good friend, a sincere student, a nice sister and a fine mate for Edward. She's also been the eyes of the viewer as the audience see the whole through her point of view(at most times). I won't say that She's been a good protagonist, but that's only because the character Bella, herself, hasn't been a good role model. I believe everyone here has their own opinions regarding the same, I respect it and accept that everyone's choice differs.
