I remember a few long running TV shows with cast writing and directing episodes.

Duchovny directed a few XFiles episodes.

Two of the leads directed episodes og SG-1, antihero wrote at least one episode.

Those two are just off the top of my head, but I remember noticing cast doing extra stuff regularly. Probably not too common in the early seasons, but it is a great way for them to branch out.

That said, some are more successful than others.