In a Pen and Paper game, wouldn't it be pretty much unheard of twenty sessions in for Bob to leave, and then Clive joins the group and then starts playing Bob's character?
It's done, if it's just for a vital part of the campaign, and the character is important. However, the normal thing would be to put Bob's character on ice in some way, because Clive would generally want to do his own thing. I've never experienced a new player wanting to "take over" a character from someone else.
