Movies are too formulaic. I just try to figure out who the shapeshifter* is.
A shapeshifter is an archetypal character whose primary essence either is in doubt ("is she lying to me?") or changes suddenly ("I thought you were a man!"). In some cases the hero is a shapeshifter, sometimes the shapeshifting qualities of a character are a central part of the story--but often suddenly in the climax you learn that someone isn't what he or she seems to be, such as when a double agent is revealed. the "shapeshifter" is a central part of storytelling, along with the hero, the mentor, the threshold guardian, the herald, and the trickster, etc., In star wars, obviously Darth Vader was the shapeshifter. So was Leia, when she was revealed to be Luke's sister. Han Solo was also a shapeshifter, since you didn't know if he would help the Alliance or not until the very end.