Although i'll concede that some guys show it up a bit the really wacky looking **** is often 100% genuine.
I've been working hard at singing for years (lol nickleback lol) and I can tell you honestly that I don't feel i'm at 100% of my ability unless i'm in a different place. When i'm really feeling it man I am gone. It's pretty funny to watch. There are whole shows where I sometimes don't really get there either but there are others that i'm gone all night long and those are a ****ing trip man.
I know several really solid guitar players who feel the same way.
getting back on topic this illustrates the point and it's pretty funny too. The Bass player about a minute in..that dude is feelin it.
http://www.youtube.com/watch?v=kW0w5FXVlBM