This might be too late...
I just played Cinderella so I guess my opinion will count for something!
Contrary to how many girls play her, Cinderella is not weak. She is a strong person who puts up with crap from her stepmother and stepsisters.
There's a line in the play that shows some background on Cinderella: "Leave my stepmother? I don't think, if Father were alive, he would like that."
I think this line indicates that, possibly, Cinderella's inclination to stay with her stepmother is based on some kind of promise she made to her father, not because she is "weak" or "dependent." Maybe she feels guilty about leaving her stepmother and stepsisters because her father asked her to "look after them" right before he died? Whatever the underlying reasoning, this line (though seemingly insignificant) definitely implies that there is some big-time father loyalty going on.
Cinderella is very "Disney Princess." Now, I wouldn't go so far as to play her like Giselle from Enchanted, but I would definitely keep in mind that the character is very graceful and elegant despite her clothing.
Cinderella is almost expected to have a certain voice - "smooth" is a good way to describe it. I don't know if you are familiar with musical terms, but try speaking in a way that is legato, rather than staccato. This is what I mean by "smooth." Her voice should also be somewhat higher-pitched. Again, not "Giselle," but it should be very feminine.
Cinderella's Vocal Range:
I believe the highest Cinderella sings is an E5 or F5, but most of her singing is perhaps in the range of C4-C#5. This is not a belting role, but at the same time, Cinderella's voice shouldn't be operatic. The sort of singing I imagine for Cinderella is sort of like Liz Callaway's voice (she did the singing for Anastasia and the Swan Princess, as well as for Jasmine in the second and third Aladdin movies).
I have pictures, info, etc. from the version of the show I was in, on my blog: http://charactress.blogspot.com
Specifically Cinderella posts: http://charactress.blogspot.com/search/label/Cinderella
Break a leg! =)