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.

I have read various interviews and such where they both have said things to the effect of, they each remember having written certain parts, and that there are parts which neither remember writing.

I believe it to be a real collaboration, where the question is almost meaningless. There are some points where we can clearly read Pratchett’s humour and some places where a bit Neil’s darker imagining shines through, but on the whole I think it is almost impossible to separate one from the other.

It will however remain on of my favourite books of all time. I can also recommend the TV series made a few years ago. It was probably as true to to book as you can make it and had me laughing out loud almost as much as the book did.

The first time through I had trouble trying to figure out who I was reading at any one point and stopped trying to guess. I might give it another go since getting through some more Rincewind and witches.

Neil Gaiman:

We were both living in England when we wrote it. At an educated guess, although neither of us ever counted, Terry probably wrote around 60,000 “raw” and I wrote 45,000 “raw” words of Good Omens, with, on the whole, Terry taking more of the plot with Adam and the Them in, and me doing more of the stuff that was slightly more tangential to the story, except that broke down pretty quickly and when we got towards the end we swapped characters so that we’d both written everyone by the time it was done, but then we also rewrote and footnoted each other’s bits as we went along, and rolled up our sleeves to take the first draft to the second (quite a lot of words), and, by the end of it, neither of us was entirely certain who had written what. It was indeed plotted in long daily phone calls, and we would post floppy disks (and this was back in 1988 when floppy disks really were pretty darn floppy) back and forth.

Terry Pratchett:

I think this is an honest account of the process of writing Good Omens. It was fairly easy to keep track of because of the way we sent disks to one another, and because I was Keeper of the Official Master Copy I can say that I wrote a bit over two thirds of Good Omens. However, we were on the phone to each other every day, at least once. If you have an idea during a brainstorming session with another guy, whose idea is it? One guy goes and writes 2,000 words after thirty minutes on the phone, what exactly is the process that’s happening? I did most of the physical writing because:

1.I had to. Neil had to keep Sandman going – I could take time off from the DW;

2. One person has to be overall editor, and do all the stitching and filling and slicing and, as I’ve said before, it was me by agreement – if it had been a graphic novel, it would have been Neil taking the chair for exactly the same reasons it was me for a novel;

3. I’m a selfish bastard and tried to write ahead to get to the good bits before Neil.

Initially, I did most of Adam and the Them and Neil did most of the Four Horsemen, and everything else kind of got done by whoever – by the end, large sections were being done by a composite creature called Terryandneil, whoever was actually hitting the keys. By agreement, I am allowed to say that Agnes Nutter, her life and death, was completely and utterly mine. And Neil proudly claims responsibility for the maggots. Neil’s had a major influence on the opening scenes, me on the ending. In the end, it was this book done by two guys, who shared the money equally and did it for fun and wouldn’t do it again for a big clock.

So from both accounts it sounds like there’s a bit more Terry Pratchett in there than Neil Gaiman, but it’s basically just considered an even split. And Agnes Nutter was 100% Pratchett.