Plinky asked, "What energizes you?"
I struggle with major depression (actually, treatment-resistant depression), so it's difficult to find much that energizes me. However, really great conversation with just a few people does it quite well. The same goes for story games, or a really good tabletop roleplaying session that focuses on characters and plot rather than hack and slash.
Being around too many people drains me of energy, and I need alone time to recuperate. A nice soak in the bath with nobody else around, time with a good book, or time to enjoy some quiet music are all pleasures I use to help recover from being drained.
I'm definitely an introvert, as evidenced by what energizes me and what drains me. I used to be able to fake extroversion for short periods of time, but I'm not entirely sure that I could do that right now.