Emma Watson was a young girl when she auditioned for the role of Hermione in the Harry Potter Films. Let's face it, it was the perfect casting choice. I don't think anyone else could have been Hermione.
However, her great acting abilities weren't always so great. When she first started out, she'd actually ruin shots by mouthing the lines from the other actors. Jimmy Kimmel dug up some old footage to embarrass her with it.