Topic: Main actors "moonlighting" in other series
I don't know if this is a new thing, but the last couple of years I have seen main actors in good shows in big and small roles in other shows. This summer season alone there is Emily Rose from Haven in Graceland and Lucas Bryant (same show) in Under the Dome. They are recurring in more than one episode. Today I also saw Rachel Nichols from Continuum in Rush, don't know if she will be recurring.
What I want to know is
1) Is this a new thing or has it happened a long time/always?
2) Is this a money issue or just for exposing an actor to different things in case their main show is over soon?
3) Are there other actors that are doing this recently (not movies, because that is a totally different thing, of course)?