Joker from The Dark Knight
Geminis are good at acting just like Joker who turns into any person. As people born under the sign can behave very differently in different situations, they have no difficulty in being almost different people. They tell a sad story about their difficult childhood and then can turn around and stab their interlocutor in the face. They quickly switch from a poor victim to a total asshole, which makes it so hard to catch them.
Alternative versions: brutal Pinhead or Billy Loomis and Stu Macher from Scream.