Initially I was going to cram all the background into one document, as I did for Bane of Souls and Journey to Altmortis. However, when that document reached 15 pages without me putting in what felt like much work I decided it'd be better to chop it up into three documents: characters, world and plot.
I really like world-building. Occasionally little details can be a bit tricky to get right, but I find it pretty relaxing and fun to put together strange peoples, old customs and so forth. Before Bane of Souls I wrote some other stuff (unpublished) and did so very spontaneously, with sod all world-building.
Partly because Bane of Souls was originally just an exercise to keep me from going rusty I deliberately tried creating a lot of background info for Highford before I started writing. This came in handy because whenever I was stuck for a name or trying to think of how a certain person would react, I could go back to my lore and it'd point me in the right direction. It also meant I had a pretty firm idea of the different peoples in the city and stopped me making stupid mistakes or quite as many continuity errors.
Journey to Altmortis required less background work because I'd done quite a bit on the Kuhrisch already (it's set in the Kuhrland) and there's not much time spent in an urban environment (unlike Bane of Souls which is 95% in the city).
The trilogy, however, is a whole other kettle of fish. There are several major cities, and I'm doing background on all of them (at this very early stage I suspect most cities will feature directly at one point or another during the story), as well as the hornskins (creatures who live on the Fettered Isles). I also need to try and work out how the mages and Church would fit into a civil war and have grappled with how much to utilise and/or show a certain figure very briefly alluded to in Bane of Souls (the original magister primus).
The cast is also much larger than I'd expected, and I've only created most of the main characters so far. Some could use a little work, but I'm very satisfied with a number of them, especially the women. I will be putting in a cast list at the end of each book.
I've got much of the plot of book one in my head but the plot will follow the creation of the world and characters. I need to have those two more firmly in place, so that the plot fits and can be trimmed to fit into a mostly realistic pre-industrial fantastical world. I did the plot after the world-building for Bane of Souls and that seemed to work pretty well.