Keira Knightley would be a convincing Desdemona because she is able to portray so much emotion when it comes to tragic deaths, and given her acting repetoir, I think she would be the most experienced to play Desdemona.
Keira Knightley plays such stubborn, tragic, passionate, and strong willed female characters. My favorite role she has ever played was Lizzie Bennett in Pride and Prejudice. She was so strong in that role. My favorite thing about her acting is when she gets fired up. She just IS passion