Jan Michael Vincent
Hollywood's history is full of actors who convincingly portrayed nasty, mean guys and in real life everyone said they were the nicest dudes anyone could ever meet.
From second hand sources I keep hearing that actor Jan Michael Vincent was the opposite. His on-film persona made him seem like one of the nicest guys around but in real-life people say that he was short-tempered and not nice to be around. It was known by the media that Michael Vincent drank a lot and this must have affected him.
All that said, Jan Michael Vincent did a lot of good movies in his heyday. This is one of them.