Discover: Character Interrogation

For the past few months, I have been banging my head against a wall trying to figure out the motives for my characters in this current story I'm working on. The basic plot elements are all finished, but the hows and whys are not answered. AND I NEED ANSWERS, PEOPLE! I guess my approach to this was a little off since I have been writing this story in first-person and thus have only been thinking about what motivates my main character...not about the people around her. 

The plot has an underlying theme of espionage, so for a fun creative challenge, I decided to write the confession of the character who is the spy in the story.

AND I'M SO GLAD I DID. Having him tell his side of the story and what he has been through in order to make the decisions he has made...it helped me understand the whole story a little more. The best part is, letting a character tell the story from another perspective, more options opened up and the plot started flowing!

Now, during my free time, I can't wait to keep writing his confession just so I can keep mining the plot-gold!

I'm debating on doing this same sort of method with a few other characters, but instead of using an interrogation as the background to get them to talk, I'm going to use a scene that is a little more natural for them. 

Obviously someone who is not a criminal, a witness, etc. wouldn't really be found needing to confess anything. The girl-next-door character might feel more natural if you write a conversation she has on the phone with a friend. Or a character you are designing just for that epic death scene in your story decides to reminisce about his life as he is being taken to heaven by an angel. 

What other ways can you get your characters to talk?