The British star - who was initially lined up to appear in the forthcoming Ridley Scott blockbuster 'Robin Hood' - is not bitter about losing the part because she believes the 40-year-old actress is "more suitable".
She explained: "There was nothing salacious. The script kept changing and evolving, to the point where I was no longer appropriate for it. Cate's 10 years older than me, which is more suitable for the script."
"If they'd recast it with Keira Knightley, I'd have been heartbroken, but they've gone for something different - and better, in my opinion. If I had the choice between Cate Blanchett and me, I'd go for Cate Blanchett!" she said.
Sienna also admitted she believes the public has a "vendetta" against her.
She explained to Total Film magazine: "There were several people supposed to be doing the film who no longer are, but because it's me it becomes this enormous drama. Is it a British thing? I don't know. I think it's a Sienna thing! There's just a vendetta against - I don't know."
Meanwhile, the 27-year-old beauty says she needs to be more vigilant with her movie role choices.
She said: "Films take two-and-a-half years to come out. I understand why I made them at the time but I'm absolutely not there now. I have to be more strategic in the choices I make."
Related Gallery: Behind The Scenes Of Russell Crowe's Robin Hood
