> Penny Lane in Almost Famous - Brad Pitt as Russell and Natalie as Penny? Now that would have been a completely different film. Crowe originally wrote the role of Russell for Pitt, which he reportedly declined for money reasons. Hudson was first to be William's sister.
> Scarlett Johansson's role in the Horse Whisperer - Horse Whisperer is the film that first put Johansson on the map. Would she still be filling the box office seats if Portman has taken this part over starring in The Diary of Anne Frank on Broadway?
> Claire Dane's Juliet in Romeo + Juliet - she auditioned for the part but was ultimately passed over due to her age. DiCaprio was 21 at the time and she was 13. Come to think of it that would look a little icky. Natalie did end up trumping Danes later for the lead in Where the Heart Is.

Source: natalieportman.com