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.
Characters are the life breath of your novel so you need to create strong, realistic, memorable characters. You want your readers to connect with them, to cheer for them, to grieve with them...to do that, you need to make realistic, memorable characters.