Yeah it's a very... impulsive event - unless you've been planning months in advance. I often found it was hard to write a spontaneous character without thinking it through much and planning ahead ><.
So far i'm around 30 pages into it, on my first draft, but I have all the lore written out in extensive detail aswell as the story progression and plot points, character bios and such in the form of various notes which i can refer to / pull from when writing. Notes are all in one big pdf document which is 500 pages long. The fact that the notes is almost as long as I expect the entire novel to be is a little worrying.
It's been a big poart of my life for a while now, needless to say.