It seems like the majority of the time, cosplayers are fat, ugly Japanophiles dressing as characters that are way out of their league. Kind of an escape from their dull, socially retarded lives into a kawaii world of pocky where the ramune flows like water.
Other people really make the characters come to life.
Either way, both groups have a ton of self-confidence - if you can cram your 300 pound frame into a Haruhi school uniform, hey, at least you're not shy.