How do Ты come up with a crossover?

How do Ты decide which programmes/films etc to crossover? Most of the ones I've seen are based on a shared actor, или occasionally just 'verses which have stuff in common and would пересекать, крест over well (e.g. two different fantasy/sci-fi programmes). But I've seen some pairings whose crossover I really don't get. They don't seem to have anything in common. How do Ты come up with those? Is it just thinking that the characters' personalities fit? или that they look like a good couple?