I change my personality depending on who I'm talking to. Offline, I'm pretty quiet and reserved, but I can be quite extroverted whenever I feel I know what I'm talking about. Typically once I get going my personality tends to default to a fairly absurd, excitable joker, but oddly, despite the fact that all my personalities trend that way, it's not who I am. It tires me out incredibly to keep that act up, but I can't really help it. Mostly I'm happy when I'm on my own and I can interact with people as and when I choose to. I guess that sounds very 'low self esteem' but that's not what I mean - I simply mean I'm at my happiest when I'm on my own, or with very few people around me. Oddly, I am pretty much always available over IM - despite me considering it a great way to talk to people (because you can consider your responses, not rushing, but it's still a lot faster than email or text) because I can drop the OTT act more easily over IM.
In person as well I can be pretty scathing, when prompted, although I'm generally very nice.