I think Camilla Belle would have actually made a good Sara
she fits the physical description better than this actress (nothing against her, she just doesnt resemble Sara in the books) and she also has a quiet way of speaking, a naturally elegant air about her, and I think she's a decent if not pretty good actor.
http://www.childstarlets.com/captures/videocaps2/amilano/pivy/ampi080. jpg Sara is desrcibed as having very black hair, greenish grey eyes (OK, Camilla has greenish brown, but close enough, lol), eyes that seem to be learning all about you when she looks at you (or something along that line), and a very conventionally pretty child. Movie-Sara is very conventionally pretty, whereas Camilla was/is exotic looking, so she's got an unconventional cuteness about her, rather than the classic one some of the other girls in the story had.
"I do pretend I am a princess, so that I can try and behave like one."