Also be wary of someone who initially seems like a perfect friend and agrees with everything you say at first... but over time criticizes you more and more, contradicts everything you say, is incredibly manipulative, and whenever anyone else is around reverts back to acting like the "nice" fictional character they initial seemed to be.

But the person I'm thinking of (a narcissistic sociopath) was also one of those people who quite early on was highly critical of others but not me. Even when I still thought she was wonderful, I thought: "If she's this critical of others, someday she'll turn that on me." I was correct.