Before you start writing, you want to make sure that you know your characters inside and out. It isn't enough just to have a general outline of them; you want to know how they'll react in specific situations, whether they're under stress, celebrating, trying to reach their goal, or anything else that might happen in the course of your novel.
If you don't know who your character is, it will be difficult for your readers to know, too. Due to this, before you start writing it is an absolute must to outline your character profiles. Now, you don't need to do this for every single character that appears in your novel, but you must take the time to outline your character profiles for all your major characters in your story. For example, I completed three character profiles for the 3 major characters that influence my story; the protagonist, the supporting character (the ally), and the antagonist. I knew that I needed to do this so that I could understand how my characters would react to each situation that they find themselves in.