For my game, it amounts to how much refresh remains and how much was spent on powers...although a few exceptions exist. RCI, for example, as shown in Changes when Susan and partner are hacking the computers
don't hex. Neither would a were. But a summer knight with any refresh remaining (equating to some of their "human soul" remaining) would. Anything wholly monstrous would not (trolls, vamps, etc) -- although I do like the idea of a "monster detector" being when all the lights suddenly blow out... A channeler would hex less than a full wizard, but would still hex. Also, and most importantly, characters will hex when it is dramatically (or comedically) important, and won't when it's not.