If you remember Jerry Lewis in "King of Comedy" with Robert DeNiro, he portrayed a real jerk very convincingly. So much so that I suspect he was playing himself.
Yep I remember it well and yes he was very good doing that serious role. That movie was supposed to star Johnny Carson and probably would have been even better if Johnny was in it!
And yes, I suspect Lewis was playing himself. For years people had insinuated that his 'Buddy Love' character was an extended rip on Dean Martin. Years later Lewis said in an interview that it wasn't true, that the basis for Buddy Love was himself when he was at his absolute worst.
Lewis takes a lot of heat but I always liked his movies. I suspect he is a dreadful person to work with and be around.