Write a letter.
Epistolary writing can add great realism and truth to a story, to your character; it can - in quick exercises - help you to decipher your characters and get into their heads.
Recently, I made my media writing students write two letters - one to a parent and one to a best friend, in which they revealed the same secret.
I was elated by how creative they became; some of them came up with insane secrets, purely hilarious secrets that entertained me to no end.
But what was illuminated the most by their letters was the sender's voice and how the sender switched up his/her style and approach depending on the audience. More often than not, students revealed only partial truth to their parents; whereas, to their best friends, they opened up their hearts and told the entire dirty story. With parents, students tended to write very flowery, longwinded, more "classy" - I'm thinking in the hopes of having the sender "sound" sincere and "prolong" the truth being revealed. On the other hand, students used more slang, short sentences, and things left unsaid in their letters to their best friends, which revealed to me that their relationships were closer, more open than with the parents.
If you're having trouble getting into your characters, take a subject and have characters write letters - to you (the writer) and to other characters.
By first letting your characters be "real" and second writing letters that speak to your characters' truth, you'll notice quirky phrases they like to repeat, a rhythm to their written speech, what they care about the most and the least, and what their feelings are toward other characters - among other things.
Three books I highly recommend if you're interested in epistolary writing: Dangerous Liaisons by Pierre-Ambrois-Francois Choderlos de Laclos, Upstate by Kalisha Buckhanon, and The Color Purple by Alice Walker.
Shon Bacon is an author, editor, and educator, whose biggest joys are writing and helping others develop their craft. She has published both creatively and academically and interviews women writers on her popular blog ChickLitGurrl: high on LATTES & WRITING. You can learn more about Shon's writings at her official website, and you can get information about her editorial services at The World According to ChickLitGurrl.