Cassandra Clare sometimes posts discussions of her characters on her tumblr in response to a question a fan has asked. I would read those and admire the thoroughness with which she seemed to understand her characters, and wish I knew my characters to that depth. My characters felt kind of shallow to me, underdeveloped, in comparison.
I do envy plotters for their familiarity with their cast prior to writing. I never really know my characters when I start. I might have a general sense of broad personality – extrovert versus introvert, for example – but beyond that I don’t know. I just start writing, and the character appears on the page as I tell the story. By the time I reach the end I know them only slightly better. There are worksheets and such you can do to try to get to know the character better but I’ve actually found these don’t work so well for me. I overthink and lose the organic-ness of figuring out the character, the spontaneity of their development that is the thing I love most about pantsing and what makes them feel so natural.
What I’ve found most insightful, however, is discussing the character with my beta readers. When one of my readers asks me a question or expresses confusion in their feedback about some aspect of the character. I really come to know my characters through explaining to someone who has read the story just how I perceived their character: why they did what they did and where it’s shown in the story. What their interests are and where this is hinted at. What their personality type is and examples to support it.
It turns out everything I need to know my characters is on the page, I just have to stop and look at it.