Really I'd say the prerequisite is that they need to be willing to keep up with current continuity so they can comment on what their own characters might do. I can get away with posters not telling stories but its much harder without them even reading them.

A couple of times when posters have said goodbye to the board they've kind of tossed me a bone and said "sure, you can use my character now I'm not interested any more." I've rarely (if ever) taken them up on it because the fun of using the character is to engage and interact with that poster and with other fellow-posters. So unless the character in question is an absolute essential classic (c.f. Lisa, Donar, Dancer) I tend to sideline them until and unless the poster's interested again.