If you've been paying attention to the credits during the last few years of South Park, you'll notice that Trey Parker has directed and written every episode. I've noticed this, too, but in my reviews I've always mentioned the two men together because the show still feels very much like a joint venture to me.
The question remains, though: what is Stone's role if he's not writing the episodes? This article helps explain. It seems that Stone, in addition to serving as an executive producer, doing voices and contributing ideas, also serves as the mouthpiece for the show while Parker works on the creative side of things. Parker would rather create than have to deal with censors and network executives, so Stone steps in to fight those battles.
Here's an interesting AP article on gay actors in Hollywood (including television) and how the industry actually seems to be trailing the public in acceptance. The notion seems to be that while the public has greeted the recent public announcements from the likes of T.R. Knight and Neil Patrick Harris with a shrug, it's still difficult for gays to make it past casting agents and producers, especially for heterosexual roles.
Here is the latest casting news from The Hollywood Reporter: