I don’t think there is a definitive answer. I know Neil Gaiman wrote the first few thousand words of what became Good Omens, but after that it gets murky. Pratchett wrote, or rewrote, a lot of Crowley, but I remember reading that they traded characters back and forth during the process. I think Gaiman has said that he doesn’t really remember who wrote what now, but he does take credit for any scenes with tentacles.