While writing a short(ish) piece in my character’s past, I realized that something had happened to him that caused some serious scarring on his arms and that he always wears long sleeves because of it. That might not seem like a big thing, but this will strongly affect my characters’ behaviour, due to anything that relates to what happened and also because he’s now hyper-sensitive about doing anything that’ll cause his scars to show. This includes getting changed when there’s the possibility of someone walking in on him, pushing the sleeves up on his shirt when it’s hot, and dodging questions about why he always wears long sleeves. Each thing mightn’t be big on its own, but they do add up to more time spent rewriting.
So if possible, know as much about your character’s past as you can before you start writing. Believe me, it will save you a lot of time rewriting the thing.