I think the best way to answer that is to list what I think a truly great character needs to have. Also, it really does pay to take time out (make space) to think about your characters in advance and plan them out. It doesn't mean you have to plan everything but you do need to know about your people in enough detail to be able to write about and for them with utter conviction. You need to decide what you need to know first!
I am convinced that when a writer writes with conviction something of that does show through in your writing and readers subconciously pick up on that. I also think they pick up when a character really doesn't work and I know, for me, when that has happened, it is nearly always due to my not taking the time to flesh my character out properly in the first place.
So a truly great character should:-
- Be Memorable (and that usually means having distinctive traits a reader will love to love or love to hate. Both work but not usually in the same character!).
- Be someone a reader would want to identify with or be happy they're nothing like them!
- Be put in situations a reader has to find out whether the character resolves or not (and how. Failure to resolve something can ironically be a resolution of sorts. For example, a character wants to achieve a goal, they find they can't do it, but they do achieve something positive they had not done before despite the overall "failure". Readers will pick up on something being achieved, a positive point of change for the character, and everyone accepts not all endings are happy ones necessarily. Endings do have to be appropriate).