Every great novel has great characters. Great characterization includes a background, flaws, habits, tics, and redeeming qualities. The characters have a life.
There are plenty of ways to get inside your character’s head. You can journal from their point of view, write a character study, or fill out questionnaires about your character. Those methods are awesome but can seem impersonal or just plain tedious at times.
If you need a quicker, more succinct way of getting inside your character’s head, you might consider writing their eulogy.