Cloud Atlas is known to consist of six primary story threads, spanning many centuries and so presumably the principals taking roles across the various segments is intended to help emphasise linking themes, rather than confuse those who miss the first few minutes. It also keeps the wage bill down for what could wind up being a bloated anthology piece. Anyway, the Herald Sun seem to have it from the horse’s mouth, quoting Weaving as follows:-
That’s a project that’s really exciting because all the actors will be playing more than one role… I actually have six characters in the same film and they are all different people in six different stories.
This could wind up being grand stuff, or it could prove to be a right old muddle. I hope I am not alone in believing that the Wachowskis have been incrementally losing their way since their high water mark of Bound and The Matrix. Whether they can find their way back to their old form, we will see.
Source: SlashFilm