Eddie Murphy is superb, especially as Jiff. He plays the character so sweat and warm, really demonstrating some great acting chops along the way.
Heather Graham was at her (brief) peak before she descended into terrible straight to DVD movies.
Steve Martin is the main focus and gets some great moments and jokes, but doesn't really feel quite like the heart of the film. That's more the cast around him.
Christine Baranski plays the very likeable Carol, and clearly enjoys showing off Carol's terrible acting.
And the always entertaining Terence Stamp eats up his screen time in a way not seen since Superman II.
Don't think too hard about the plot, just enjoy the ride, as well as the film within a film: Chubby Rain.