Let's say you're a fanfic writer. Let's make another assumption and say that you like a particular actor and everything they've ever been in, because they were in it. It's entirely possible that you may write a Doppelganger Crossover, a story in which two characters played by the same actor meet each other. Or that they are each other. In other words, a crossover which is only happening because the two sources share an actor. On occasion, this can be one part of a Massive Multiplayer Crossover. Compare I Want You to Meet An Old Friend of Mine.