This is your protagonist speaking...

Write your novel. Use my viewpoint. But first you have to get to know me. Every little thing matters - but it might not make it all on the page. Will I tell someone my middle name is Gertrude? My least favorite animal is a sloth? Secretly all I want out of life is to get to the end of my book still alive...

Seriously, though. This book project we're embarking on has so many facets, and one of them needs to be a great character that we can root for (or hate, depending on what you're writing). Your character has external goals (the things they think they want to do) and internal flaws (that they need to overcome in order to get anywhere in the story). This is the time to explore that character for all they might be.

Haven't figured out exactly what the story is about or who your protagonist is? Start brainstorming different characters that keep your interest. Remember, you have to hang out with this character not just through the Prep, but all of NaNo and any edits further down the line. Make it someone you want to spend that kind of time with.

If you're in a series and you think you already know your protagonist? Look for the things that aren't on the page. Make sure those flaws keep coming (unless you're writing a James Bond or Batman style character who always has the same flaws and never really overcomes them) - and be real about what that means. You can also fill in backstory and try to see how that impacts the character's actions through the coming novel.

Here's a few links to help.

https://www.masterclass.com/articles/how-to-write-great-characters

https://blog.reedsy.com/character-profile/
(I think some of that might be overkill. Maybe try just a few.

https://writingcooperative.com/3-types-of-characters-that-could-lead-your-story-...