Just because I don't write a certain story doesn't mean the characters will never see the light of day. In fact, a lot of the time I come up with a setting long before I know who's in it, and in times like this, I take a look at some of my "abandoned" characters to fill the void.
For example, the current cast of New Fiction Idea #40 consists of Cahokia and Axum from Man in the Blade and Dobson and Crick from Mount. Given the totally different universe, they're not precisely their old selves, but what fun would it be if they were?
In another vein, I've taken fan characters for fan fictions and turned them into fantastic creatures for OCTs. How would this character react to having powers? What powers would fit this one? What powers would be the worst for this other one?
It's really quite similar to writing Alternate Universe fan fiction—take a character from one place, put them in another, and see how much changes or stays the same. It may seem a bit like cheating or copying, but there are only so many character traits out there. Why try to put together an entirely new set every time when you can just take an old friend or archetype and tweak it?
And some of my characters are just too dear to me to be left behind altogether. If I can't write them in their original setting, why shouldn't I pick them up and put them somewhere else? I think they deserve a place in my stories outside of my head, and I'll try not to make them wait in line too long.
How about you? Copy-pasting any characters of your own?