I would say, you have at least a little knowledge and background in what your character does. While you can play a character that is completely out of your depth of education, knowledge, and experience, it's much harder than if you have at least some basic idea what the skills and background of the character should be like.
The second thing I would offer is that you should show some empathy towards how that character would act. For example, you should think hard about say a character that is of high social standing, and has a background of being an outstanding citizen and say military officer willingly doing some overt, seriously, criminal act. Or, the reverse. You should have some idea what your character's background is and how that would make them act.