A follower when I'm around people I don't know. I have 0% leadership skills anyway, and people tend to have better ideas than I do. As well as that, it's just a lot easier to do what people tell you to do instead of having to think about what to do all the time. Less responsibility, you know.
When I'm with my friends, I lead them into all sorts of stupid situations.
I do both, if there is a need for a leader I will step up but otherwise I'll happily leave someone better to do it. I've dealt with stressful situations before and managed large groups too.