I've done a lot of different things, trying to get a firm understanding of my MCs. I've filled out pretty much every questionnaire I could find, most full of trivial details. I've got concept pictures of them. I've worked out their past lives. But I still couldn't quite understand them.
I looked over several psychology/psychiatry books (because, let's face it, when I don't understand something I turn to science), but I still couldn't find what I was looking for. My main characters aren't crazy. Books on psychosis and biopolar disorder and borderline personality disorder didn't help. I began to feel like the thing I was seeking was so ordinary, so commonplace, that everyone else automatically understood it. What was I missing?
Then I remembered something. The Myers-Briggs test. The Myers-Briggs Type Indicator (MBTI) is a psychometric questionnaire that is supposed to examine differences in how people perceive and make decisions about their world. Whether they're more rational or emotional. More introverted or extroverted. Based on the work of Carl Jung, the MBTI "focuses on normal populations and emphasizes the value of naturally occuring differences." (Wikipedia)
Which was exactly what I wanted.
I've taken the test twice so far, one time for each of my protagonists. But it was so helpful that I'm considering taking it as my antagonists, too.
There are four areas that the MBTI measures and for each area, there are two options. The first is Introverted vs Extroverted. Intuition vs Sensing. Feeling vs Thinking. And Percieving vs Judging. There are 16 possible outcomes of the test, and each one gives a lot of information about how the person handles energy, information, decision-making, and their lifestyle.
For instance, one of my characters tested INTJ, meaning she's introverted, intuitive, thinking, and judging. This tells me that she prefers one or two close friends and quiet time. That she's good with abstract thinking, but not necessarily at focusing on current situations. She values objective criteria, but doesn't have much use for social considerations. She also likes to have a sense of control which can seem limiting to other people.
That's incredible! Now when I write as her, I know that focus will be something she struggles with. That she uses memories and speculation about the future to understand the present. That she might get overwhelmed by a crowd or be severely uncomfortable when she loses control. Hopefully, I'll be able to translate all of this information into organic, sincere writing because now I understand how she thinks. Hopefully I'll be able to fully get into her head.
The test is only 72 questions and you can take it for free here as many times as you like. After you get your result, you can look it up on Wikipedia for more information (just type INTJ or whatever your result is into the Wiki search bar). It'll also give you historical and fictional examples of other people who are the same type. Typelogic.com is another site that will give you a lot of info.
If you're having trouble solidifying your characters, it can't hurt to give this a shot. It worked for me. Let me know if it works for you!
Also, I found some fun charts showing MBTI types for the characters of Harry Potter and Downton Abbey. (I'm Hermione and Tom! Although occasionally I test as Luna and Matthew.)