Munday:

Tell us about a time your characters surprised you. Maybe you thought for sure they'd react a certain way to a certain situation, or that a plot would happen in such and such a fashion... and it just didn't. What came out ended up making you go 'huh!' or 'whaa?' or 'Wait no you're making me have to change the entire novel!!' but either way, you weren't prepared for what your characters said or did.