I want to say that I wing it, but thinking about it I probably do work it more...
It's more a case of "Ok, this character will show up in the next page, maybe I should get around to that character design."
I usually have a clear idea of the concept in my head, but do a bit of research on clothing since my comic is set in Victorian England.
Wait, that does sound like winging it... Okay, a bit of both then?