Having not done a series yet, I’ve not had to tackle the idea of a rotating cast. With each book being a one-off, the cast is the cast is the cast, but I’ve been reading the Fables comic books again recently, and that’s been going on for long enough that rotating the cast happens organically.
When this sort of thing happens in TV, it is almost always awful. Rarely does it happen because the story demands it, more likely it’s because the actors had better jobs to go off to. The replacements show up and something just doesn’t feel the same.
Maybe this is one of those things where stuff just works differently in TV than in comics or books. Or maybe it’s a problem in all media if you put more effort into the characters than the world. The reason the changing of the guard works in Fables is because the world is so rich, there’s room for infinite numbers of stories and characters. It feels like a world that would go on without a specific character. If your world is centred entirely around one character, the world is going to collapse when you remove him.
I’d quite like to craft the sort of story that can keep going even if I change the guard a little. Maybe I already have.