I believe there are two kinds of fiction writers – the ones who imagine a plot and then fill it with characters, and the ones who imagine characters and build a plot around them. I’m definitely the latter , which is sort of weird considering I’m really not much of a people person.
Creating complex characters inside your head can be tricky. You basically have to know these imaginary people better than you know anyone in the real world. They will eventually give up all the sorts of secrets that real people keep buried for their entire lives. It takes time though. You can’t truly know anyone in just a day. Keep listening to them, gain their trust, and soon they will open up to you.
One thing I like doing to help characters come out of their shells is to fill out surveys for them – kind of like the ones you filled out with your friends as a teen but less obnoxious because this one isn’t all about yourself. I found one on Tumblr that I quite like:
Some of the questions kind of overlap, but you can adapt them to suit your needs. You might even think of some other questions you need your characters to answer!