Kristen Stewart has naturally green eyes. She wears brown contacts for filming because Bella is supposed to have brown eyes.
Kellan Lutz intended on keeping his natural curly hair for the role of Emmett Cullen. However, his previous role in "Generation Kill" (2008) (mini) required a buzz cut and his hair didn't grow fast enough for the filming of Twilight.
Henry Cavill was Stephenie Meyer's first choice to play Edward, but by the time the film came to be, he was 23 years old, and no longer looked the part of a 17-year old.
For the role of Edward Cullen, Robert Pattinson flew from England to director Catherine Hardwicke's house to audition. There, he and Kristen Stewart, who was already cast as Bella Swan, rehearsed the "Love Scene"/"Meadow Scene" on Hardwicke's own bed.