Keeping track of characters can be a challenge. Even names are tough when writing more than one novel at a time. I know I've used the wrong name before and needed to fix it in rewrite. That is one good thing to consider when going through the novel to edit. Checking character details when described.

There are character sheets or questionnaires available online. You can pick from the basic to the very extensive. We used some for NaNoWriMo prep though not all sections are relevant depending on story and age of character.

Or there is the "interview" the character. Can be fun for some, as has been mentioned. I don't like doing that but have given it a try to at least see if it would work well for me. My biggest issues is that I don't describe the characters much. I don't list eye colors and specific hair details unless it seems absolutely necessary. While I know some really want those details, there is also the risk of it coming off force, unnatural and like a list at times. Cause honestly, I don't notice those details. And in romance some things are way over focused. Like how much character's eyes dilate or pupils get blown during kissing, etc.

Sometimes I like to leave a little room for a reader to build an image in their head closer to themselves for the characters instead of giving exact details, but I do know I could be more descriptive in my writing. It's something to work on.