I wonder why they dropped Paul to voice Roger? This was done long before his arrest for "indecent exposure" and around the peak of the Pee Wee Playhouse run. I'm thinking that perhaps he was too busy to do this and his television show.
Read the book, it is much weirder than you could possibly imagine. The cartoon characters talk in word balloons that fill actual space. Jessica Rabbit's seedy past is in Tijuana Bibles and she's being blackmailed by a character named "Dr. Beaver". Judge Doom and the Chinatown rip-off plot are absent entirely.
The author raised a bit of a stink about the movie completely changing the original plot of his novel; but then the film was a smash hit, and he was only too happy to write a "sequel" which followed the movie's continuity and dismissed the original book as a dream.
The tone of the book was entirely too dark, it's a semi-serious send-up of Raymond Chandler. Roger literally gets murdered. That's not a spoiler, that's the plot of the novel, Roger's cartoon ghost urging Eddie Valiant to solve his murder before it dissipates into nothingness. Changing the tone and plot was a risky move that actually worked.