I was an extra on the set of Idiocracy. To my recollections, the coolest actor there was Terry Crews. He was funny off camera, engaging, and once openly asked how come the extras have to eat lunch separately from the main cast and crew. I also shook hands with Mike Judge, very cool guy.
I wouldn't go so far as to say he was an asshole but least personable actor there was Luke Wilson. One time as he was walking back to his trailer, I called out his name to get his attention and waved. He waved back, but with a sneer.