In books and articles about writing a lot is often made of whether someone is a planner, or just gets stuck into the writing without knowing the characters or plot, or where they are going to end up. I tend to think of it as being somewhere on a continuum between the two, rather than one or the other.
To a certain extent I am a planner. I have an initial idea of a situation, soon after that the main characters appear, and I leave it in the back of my mind bubbling away, visiting every now and again to see what else has happened until it gets to a point where I need to start recording it.
I make notes about the initial conflict, the main turning points, some scene planning, and do a little work on the character spreadsheet.
I said I am a planner to a certain extent. That extent tends to be about half of the novel and then I get stuck. I wallow around for a few days wondering what else is going to happen and then decide to start writing. In the process of writing I get to know the characters much better and they move the story, not me.
So when I first start writing, my character spreadsheet is only thinly populated, the rest comes as I get to know the characters. When I discover new information about a character, I add it to the spreadsheet at the end of that writing session. The same with new characters.
This means that later if there is something I can’t remember, or want to check, I don’t have to search through pages of manuscript, but can find it easily in the spreadsheet. This is especially useful when a timeline is important to the plot. How old was X when this happened? When did A & B first meet?
As you can guess from the comments above my character sheets inevitably get a lot of other information added to them during writing, but it does mean there is one place to go to find this information later.