When you are so used to having that insight, it is hard to take into account that Kristen can't ever be as explicit in terms of what she is experiencing as Bella as we would like. It's not like we can always have a running narration like we get in the book. Also, I honestly believe Kristen is handicapped somewhat by the way SM writes Bella's selflessness/selfishness. Many of Bella's reactions are completely at odds with what we know her to be feeling because she is trying to cover up her fears, or her doubts, or her overwhelming reactions.
And that's a problem with book to movie adaptions-the whole story doesn't always translate. Things can be condensed and explained in shorter periods of time, but some things do go missing. Just go back to Twilight where Kristen recites some of the thoughs that Stephenie wrote that Bella was thinking. And also watch Speak where Kristen does the same with Melinda.
That aspect is limited, but I do feel that through her actions that Kristen does a good job of trying to portray what Bella is thinking and the way that Bella is trying to sheild some of those thoughts. One must try to remember that Bella is kinda a weird character-she's oddly scared but extremely brave at the same time; she's scared of Victoria because of what she worries that Victoria could do to her family and friends, but she's not afraid of Edward or Jacob when they try to insist that Bella keep her distance.
Bella's a strange character in that way, and especially in Twilight, Kristen really didn't have a crap load to go off of aside from what she read in the books and SM's stuff, the former being one of the few failings(not really a failing, but still somewhat of a problem, especially for the cast) of the novels-great story, and it's good that Steph left things about the characters open to interpretation for the readers, but, at least early on, character development wasn't the novels' strongest point. But then again, leaving the characters open to the readers' intreptation is a double edged sword; it inspires readers(and the cast of the movies) to have an imagination, but it kinda short-chains or befuddles the cast, and it's known that they might not match up to everyone's expectations, because their picture of the chacters can vary so much.
All and all, everyone, especially Kristen, Rob and Taylor, pretty much got it 90-99% right, but no one's perfect, just like how the novels aren't perfect, but we love them anyways
. That, and they can all act, inspite of what the shippers and jealous people say, as KStew has taken more crap than any 19 year old young lady should for doing something that she likes for her own reasons.