I think Ben Stiller was the only American who came off as "pompous and self-important", but he was clearly making fun of the quality of his own movies, like when he was extolling the healing powers of Dodgeball.
Sam Jackson's behavior seemed reasonable, during the race flap on set. And Robert De Niro was not only willing to meet with Andy, but was amused by Darren's naked-lady pen, and went to a pub with him after. Am I missing any other Americans?
I think the most reprehensible of all the portrayals was Clive Owen, although Bowie's mockery of Andy was particularly cruel.