Out of all the details of South Park: The Game revealed in a GameInformer interview with Trey Parker and Matt Stone, the most important is their direct participation in the adapted full-scale RPG.
We've already mentioned that they have written the script and the dialogue, but they've also given Obsidian 15 years' worth of assets and even a detailed list of approved textures and colors. They even created an entire map of South Park (none existed before) so that the developers could nail down a town hub. They want South Park: The Game to succeed as much as we do.
(A quick note: Don't read the rest of this article if you don't want to be spoiled, though the details on the story are still vague.)
The player will begin the game as a silent protagonist, a new kid on the block who's trying to fit in with the other fourth graders in South Park. Eventually, the player gets wrapped up in a LARP that the neighborhood kids are playing and of course the game turns into a real epic adventure. Eric Cartman introduces you to the game and asks whether you want to be a wizard, paladin, adventurer, rogue, or a fifth class of his own evil creation.
Similar to Paper Mario, players can initiate combat by striking foes in the field first. In battle, well-timed inputs will result in higher attack damage and defense. Obsidian will be incorporating dynamic sequences so that players don't watch fixed animations too often and will be using the Dungeon Siege III engine to bring the South Park world to life.
South Park: The Game is scheduled for release some time next year. (I imagine some time after E3 2012.)