Do you ever struggle with character interactions in your novel? When creating characters, I think most writers understand that they need to think about them in terms of physicality and personality. After all, your readers need to have a vague notion of the characters' looks and attitudes in order to connect to them. But it doesn't end there, interaction is also important and needs to be considered.
There are a number of things that can drive readers away from your book, and badly-developed characters are definitely close to the top of that list. It's not always done intentionally but it's something that can ruin a good story. So take the time to learn how not to write a Mary Sue-type character.