True.. I'm guessing it was a artistic decision, as it made Jo Jo seem that much lonelier in that he didn't have any real friends during his time in Hollywood (at least in the movie).
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I didnt quite mean the actual Paul Mooney being in the film, I meant why didnt they have an analogue(fictionalized version) for him as a character like Jojo/Richard.
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When there's no more room in Hollywood, remakes shall walk the Earth.
'

I know this is kinda late.. but he does have a small non-speaking role in the film. You can see him at the club where JoJo meets Debbie Allen's character. He's sitting a a table next to Richard.
Maybe he didn't want to be in it.. he's never really had that many acting roles. He might be more comfortable behind the scenes.
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