It's probably important to mention that everyone has their own way of developing characters. Sometimes it's a mix of both things. Carefully thought out and planned one moment and then spontaneous the next. I have to say that characters often can make themselves, though not all the time. In a moment of writing, at times you just find a new aspect of that character you hadn't considered before. It's almost as if it wrote itself. I think everyone has their own balance of the planned and unplanned. It's also a matter of testing things out and finding what works best for you. What works for one person may not for another.