I have a friend who has recently begun trying his hand at writing fiction. He wants to write character-driven fiction. He believes--and I agree--that the best fiction has deep, round, believable characters. The trick for the writer is knowing how to create characters like this.
Authors get asked a lot where their characters come from. Surely we must know someone just like this character or that one, and we patterned the character after him. This might be true for some writers but it's never been the case for me. As a matter of fact, that's the death of a character for me: I can't just fictionalize someone I know. They always come out boring and wooden. Ironically, the character has to come completely out of my imagination to develop the qualities of a real person.
I told him that the way to develop your character is to put him or her in a situation and see what they do. Then do it again and again and again. At a certain point, the character 'clicks': he is suddenly real to me and I know what he will do or say in any situation. At that point, he or she is like a real person to me.
There was a time when Lanny, Jonathan, and Adair were as real to me as my brother and sisters. I'd spent so much time with them--ten years for the first book, another three for the next two, every day spent following them around and chronicling every single thing they did. As I wound down the books, I started to miss them. I started to panic, too, wondering who I would hang out with. Maybe not every writer is like this, but I spend so much time by myself that I don't have much time to spend with real flesh-and-blood friends. I liked Lanny, Jonathan, and Adair. They were interesting, they were fun. I wondered who was going to take their place.
Back to writing advice. Some people believe you should interview your main characters to get to know them better. That you should ask them all sorts of questions that will reveal their true natures to you. It doesn't all end up in the book of course, but some of it might. Some of it might lead you to find out really interesting things about your character that could deepen your story--for instance, an aversion to heights that gets woven into the plot. Just don't be too hokey or transparent about it.
Here's an example of such a questionnaire: this one is a list of questions that Marcel Proust was asked about himself. And if that seems too precious, here's a list from the Gotham Writers Group.