Donald Maass, in his Writing the Breakout Novel Workbook, talked about figuring out what a character would never do, then putting them in a situation where they have to do that exact thing. Breaking through your character's inhibitions can create some powerful scenes. For example, going back to Darkwing Duck (what can I say? I love those Disney cartoons.) in the two part pilot episode, Darkly Dawns the Duck, Darkwing tells us that he never works with anyone, and he never takes off his mask. By the end of the pilot he's accepted a sidekick, adopted a daughter, and taken off his mask -at least during the daytime. He's a stronger character for having moved past his self-imposed isolation.
It reveals what a character is made of when you show why they'd change their mind about doing things they insisted they'd never do.
My problem is that when I set down to list things my hero would never do, it's easy for me to come up with extremes. Things like, My Character Would Never: A) blow up his home planet. B) assassinate the president. C) steal candy from babies. Then I stare at my list and trying to think what would make my hero suddenly decide to blow up planets and steal candy from babies... and wonder if my hero even qualifies a hero anymore, and if maybe I'd better forget this whole `reveal what your character is really made of' thing since I'm obviously doing it wrong.
It's the same problem I ran into when I tried Maass's exercise of thinking about the worst thing that could possibly happen to my character, and then making it happen. I'd heard a few too many stories from Fox's Book of Martyrs in Sunday School as a kid, so no matter what character I used for the `worst thing' question, the answer always involved whips, brands, and possibly the rack.
To do the exercises right, I finally had to tailor the questions to eliminate the extremes. `What's the one thing my character would never do besides blowing up planets?' `What's the worst that could happen that doesn't involve torture?' It's easy to think of things that no one is likely to do, or that anyone would consider awful. It takes a lot more work to invent the kind of character who would never work in a knitting factory and to whom the worst thing that could happen would be public speaking. Once I figured out the trick to it, those exercises were a great way to build characters.