Ghost Writers (1958)

Two writers at Paramount Cartoon Studios try to think of good stories for Casper the Friendly Ghost. They review scenes from "Casper's Spree Under The Sea," "Once Upon A Rhyme" and "To Boo Or Not To Boo." Eventually Casper himself emerges from a poster on the wall and scares the writers away. Casper explains that he thought up that gag to end the cartoon!