i put down "other." lois and clark live in my head. i give them a situation, they react to it, and there you go. it happens to a lesser degree with martha, jonathan, and perry. i think i've seen enough of pretty much all the main l&c characters that i can get a good feel for style. since i tend to focus on l&c, that's more or less all i need. if i have trouble writing someone else, well, they conveniently manage to not show up in the story that much.
it's the other characters that i have trouble with, the ones i don't know very well, especially the ones i have to invent myself. i think that may be the hardest part. maybe.
the thing that i really have the most trouble with, actually, is the plots. i can come up with ideas left and right, and i can usually come up with the people i need to make them work. i can come up with a few things that i think should happen and a general direction, but i have a devil of a time trying to flesh out the plots. that's why my average fic length is less than 15k...
Paul