Its like, more of a symbolic/metaphorical thing than we think. Palps shows his true face, reveals his true nature, etc when he's fighting. He had been trying to hide his true identity/ies from everyone. Anakin, likewise had been trying to hide all his dark feelings and such from everyone. He was Darth Vadar, but maybe he didn't entirely want to be, the fight was the climax that brought it out almost if not entirely.
2 cents