The idea of Kevin Smith directing a movie called Zack and Miri Make a Porno is a perfect fit for most movie fans.

Below, Smith answers a question from ComingSoon.net about how he casted the comedy:

At what point did you decide you wanted to have [Seth Rogen] and [Elizabeth Banks] star in it?
I started writing the script with Seth in mind. It was written for Seth. I saw him in "40-Year-Old Virgin" on DVD, 'cause I didn't see it in theaters, 'cause we were making "Clerks II" at the time, and when I finally caught up with it on DVD, I was like, "This dude is genius!"

I like that movie a lot, and Carell's wonderful, but Rogen was the true star to me, and I was like, "This dude needs to be a leading man." So I started writing it and handed him the draft two weeks before "Knocked Up" opened, at which point I was like, "This dude is a leading man, like this guy's on billboards all over town." I was a little afraid we were not going to get him at that point.

When you add to that the fact that he can generate his own material, he certainly doesn't need anyone writing for him, but mercifully, he liked the script and jumped on it.