Demi Lovato
Before she was a Disney starlet or mega-pop star, Demi Lovato was just another actress. In Grey's Anatomy, she played a patient with a disease no one saw coming. Her role, Hayley May, was a girl who's mysterious condition made her want to claw her own eyes out. Eventually the doctors figured out what was happening to her, but not without some distressing and unsettling moments from Lovato.