Sounds weird right? Write fanfiction so you can novel, sounds counter productive.
And I’ve yet to find a better way to get to know my characters than to put them in a pre-made fictional world like Merlin or Harry Potter or something more story appropriate and write a crossover. Most of the time this takes place post action of the story for me.
Take my current WIP, I’ve got a Harry Potter crossover that’ll never see the light of day in the works. I’ve discovered that Reg is a total sarcastic asshat when he wants to be; and Tanner and Carson, friends of the family, totally fuel his sarcasm. I don’t know how Topher put up with it for going on twenty years now. Then again, sarcasm tends to just run strong in my characters.
The point is, writing crossovers with similar fandoms has not only helped me get to know my characters, it’s inspired me in different scenes.