I'm a very emotionally flat person in real life, and the way I act in reality is relatively unique. The result of this is that every character I've ever made has been painfully recognizable as me. Even if I try to put a spin on it during character creation, in practice it doesn't play out in a meaningful way. I really don't want to be "that guy" for the sake of others and myself. How do I give my characters a personality of their own without becoming a bad caricature?
This is an intense question. Some people naturally can role play or act just as others are more adept at sports or playing musical instruments. This is a serious suggestion but I would recommend acting workshops if you truly want to learn methods of "becoming" different characters. Acting is a skill like writing or archery that can be developed with training.