Long story short - I don't think Keira Knightley is a particularly good actress (she seems to use the exact same mannerisms for every single role I've ever seen her in - and while they were charming in "Bend it Like Beckham", they're getting less so) and I think she's completely overexposed - there are other very competent and very pretty British actresses who, I feel, would do better as Cecilia. For example, someone like Rachel Weisz. YM obviously varies.

Also, I don't think Briony is the "main character" so much as the "author" - in that sense, I suppose she's "main", but I don't usually think of someone who is an observer to the degree that she is as being the central character.