The book did feature cameos of several other characters, yes -- although since the setting and plot revolved around comic-strip characters (hence the "toons" speaking with actual word balloons appearing over their heads) rather than animated cartoons, the cameos were characters like Snoopy, Hagar the Horrible, Dick Tracy, Blondie & Dagwood, and so on.
The change from comic-strip characters to animated characters was primarily because while everyone involved loved the idea of a film-noir murder mystery in a toon/human world, they realized pretty early on that while having the toons be comic-strip characters speaking in word balloons worked well enough in a printed novel, it would never work as a motion picture.
The book did feature cameos of several other characters, yes -- although since the setting and plot revolved around comic-strip characters (hence the "toons" speaking with actual word balloons appearing over their heads) rather than animated cartoons, the cameos were characters like Snoopy, Hagar the Horrible, Dick Tracy, Blondie & Dagwood, and so on.
The change from comic-strip characters to animated characters was primarily because while everyone involved loved the idea of a film-noir murder mystery in a toon/human world, they realized pretty early on that while having the toons be comic-strip characters speaking in word balloons worked well enough in a printed novel, it would never work as a motion picture.