I'm involved in a Facebook RP group. You basically take your characters and throw them into unfamiliar situations and see what happens - in the case of Jay [from the magician series] I ended up exploring him at the age of 15 (roughly 500 years after the present in the stories) and wrote some stories about him in that era, and still need to write a couple more from when he's 21. It definitely improved the character and I always have too much fun with Jay. Plus there ended up being a relationship with someone sad, and Jay wanting to help their sadness despite the fact that Jay is basically asexual. Made for a lot of fun and a neat way to explore the character.
Plus Jay got to use time travel as a form of self-help on himself. Because Jay. And it makes for a hideous amount of sense.
For Grimoire, I went with Trudy. Because she was the only character - in form if not in name - to remain from the 2010 first pass-through of the story idea. I knew she was in her 40s, was Protected (and what by) and used to be Someone. I knew she couldn't see properly, and used to be a Seer. And that was it. So I threw her into the game at the age of 16, a few months after she makes the Deal. I figured out her backstory, some of what is going on with her ... and she fell in love. Which I never considered, but the other player and I surprised each other with it, ran with it and it's been nicely weird. She may be a secondary character in the novel, but she isn't in her own life and her history now has a fun amount of depth to it that it never did before.
I am not quite sure how timelines line up, or even what becomes of Trudy and Quentin - he can't come into her universe at all, pretty much, and some day the fae do call her home, but it's neat to explore along with her altering vision and her being pissed off about it. Not the lack of sight per se, but the fact that being still a sort-of seer and sort-of blind made her a cliche and she's utterly livid about that :)