Why is Jerry Lewis in this movie?

maybe he's friends with someone who worked on the film? That's the only reason I can think of. I definitely would have liked to see more of his character though, and more of Nic Cage's character's everyday life. Just a little more to these characters, showing what made them tic, would have gone a long way I think.

Actor playing a part in a movie shocks Hollywood.

Its a Me, Mario - https://www.youtube.com/watch?v=6SL-aes_inM

It was a little weird. Maybe Nic and Jerry wanted to work with one another?

Actually, I just googled it. Jerry is Nic's idol, they met and became friends, Nic called Jerry and asked him to be in a movie with him to satisfy a dream:

EXCERPT from interview: Cage and Lewis stayed in touch after that initial meeting. “Then I sort of relocated to Nevada,” says Cage. “I found out we were neighbors for a moment in time [and] I would invite him out to dinner. When The Trust happened, I said, ‘Jerry, I don’t normally want to put people on the spot but one of my dreams is to be in a movie with you. I’d love to construct a part where you and I can be on camera together.’ And he said, ‘Absolutely.’ Which was like, ‘Wow!’ So, he did us the favor. He played the part of my father in the movie. For me, it was one of the greatest days of my life and certainly the greatest day of my career.”

I couldnt figure that out either.