Can a directer Star in his or her own film when someone else is Producing it?
They can star in it if they're producing it themselves, so yes they can can when someone else is... there's no laws that forbid it
Yep, quite a few directors do it, if not just for a cameo. Think Hitchcock and Quentin Tarentino.
mel brookes stared in a few of his movies he made and all the other ones he did cameos
yeah, I think that was the case on Sabrina the Teenage Witch