The exact meaning will depend on the context. But the general concept of "a rotating cast of .. " means that it is not necessary for one person to always play a specific role. Different individuals may come and go and play the same role.
A real life example could be a band that is set up as a rotating cast of musicians. So, drummers/guitarists/singers/etc are free to join and leave and whoever are in the current roster will perform using the band's name.