I find a great thing to do is give them an inherent weakness. Most people have some neurosis or other that drives them to make the decisions they make, for good or bad. A good place to start is with one of the seven deadly sins. It also gives you a great premise on which to base your characters development - for example, an avaricious character might learn what it is to be poor, an envious character might learn what it is to be selfless. I find it gives you a framework in which to explore the characters responses and growth throughout a story.
i tend to make them fill out a job application, that way i get to know them in a professional sense, then do like an informal interview to get to know them better
Working smaller in character development and giving them habits and rituals always works for me. Defining their smallest idiosyncrasies and the way they do something rather than just the action.
For me I tend to develop my main characters the most. These are the ones who get the most back story and history from me. Then there are also the main supporting characters, these characters can also get some history if its necessary to the over all plot of the story. As for supporting characters or minor ones I come up with very little or not history, and maybe a little back ground. But there is no real need to give them huge back story because they are only there to support plot not drive it in any way.
I have two methods that work really well. The first is talk to them. You can interview them, or you can get them to write a monologue, it's really up to you. But getting them to respond in their voice really works. This isn't to put into your writing (it makes horrible prose). It is for you to get to know them. Let me give you an example. "What was your relationship with your parents like?" "Everyone seems to have some problem with their parents but that wasn't it with me. I had good parents. They loved each other, they loved me. My Dad wanted me to grow up to be like him, but I wasn't like him at all. Yet he seemed to accept that. If I'm honest, I put too much sway by what my parents think." You get the idea. The other thing I like to do is relate to them with my own experiences. This makes them feel more real. This includes bad guys too. So for example, my character had their house robbed. I could have them crying on the kitchen floor, but that just feels two-dimensional. Have I had my house robbed? No, but my brother used to steal my pocket money. I was a saver, he was a spender. My money would disappear and I was certain it was him, but I couldn't be sure. Maybe my character feels they know who it is as well. They start to be suspicious of them and try to set them up to make a fatal error. Suddenly my character feels more real. Does that make sense?
Yes I have been thinking about the interview method for a while as well... but for some reason I have never used it because it seemed boring. Going to give it a try anyhow!
Not sure I can be much help here. I know my characters from the outset. I know who they are and what they do and a little of what motivates them. The rest, just as with any other relationship, you learn as you go along. You meet somebody new and you don't know everything about them at the end of that first meeting. The same is true with your story characters. The more you spend time with them, the more you learn about them... and yourself.
Whenever I create a new batch of characters, I always take a few days and write out a few scenarios for each main character. You really have to put yourself into their position to see how they would react, not how YOU would react. It's difficult, but if you can abandon your own way of thinking and instead, in essence, become the character, it can be very beneficial. Once I've written a few for each I'll go back and read them then create a personality sheet/bio if I think it's necessary. I usually only do this for really important characters though. With lesser characters I just give them a few traits and react to situations as they arise.
