Sometime around 1987 Neil Gaiman started writing a short story about William the antichrist, featuring an ineffectual demon called Crawleigh who drove a 2CV. He handed a few copies of the first draft out to friends and was promptly distracted by created by creating The Sandman series. About a year later, Terry Pratchett rang him up and asked what he was doing with the story - the rest is, as gets said, history.
Written over half a year the story seems a true collbaorative effort, with the two authors passing drafts back and forth. As Gaiman himself has said:
People still ask us who wrote what, and, mostly, we’ve forgotten. We tried to make sure that by the end we’d each written all of the major characters (I handed over the Four Horsemen to Terry when they got to the air force base, and I took the Them). There were bits we were both convinced we had written, and bits we were both convinced that we hadn’t.
Now, if you’re interested in Stylometry, the statistical analysis of variations in literary style between one writers and/or genres – essentially the author’s literary fingerprint – then you’ll enjoy the analysis of Good Omens that Elizabeth Callaway has done.
To quote her;
The pattern below the horizontal white line represents the signal from the author to whom the program attributed the majority of the authorship (Gaiman is in red and Pratchett is in green). The top, fainter pattern roughly shows how much signal there is from the other author. Together they add up to 100% in each section of the text.