So when I see a movie with a credit "based on a novel by" or "based on a stage play by" I should assume the dastardly forces of Hollywood are "by no accident" trying to steal a "creator" credit from an author or playwright?


You may, if you like -- but the more likely explanation is that this specific language, carefully chosen, is based on a legal recommendation, for specific reasons.

A Week Ago Tuesday: My blog, where I never, ever write about comics (though I think I did, once or twice).