I wrote the first 5,000 words of William the Antichrist. It had a demon named Crawleigh. He drove a Citroen 2CV, and was ineffectual. Proper demons like Hastur and Ligur loathed him. It had a baby swap. I sent it to a few friends for feedback. Then my graphic novel Sandman happened, and it was almost a year later that the phone rang.
"It's Terry," said Terry. "'Ere. That thing you sent me. Are you doing anything with it?"
"Not really."
"Well, I think I know what happens next. Do you want to sell it to me? Or write it together?"
"Write it together," I said, because I was not stupid, and because that was the nearest I was ever going to get to Michaelangelo phoning to ask if I wanted to paint a ceiling with him.

Good Omens: How Neil Gaiman and Terry Pratchett wrote a book (2014)