[bobbyi_settv]

Until very recently, I thought they wrote all the episodes without outside involvement, but then I recently came across this clip where Bill Hader talks about being part of a "South Park retreat" where they bring people to brainstorm episodes

https://www.youtube.com/watch?v=XdZnGz9CWpU

[greggh]

Bill Hader was on the writing team, he worked in the office during the seasons. There is a documentary that shows what its like there, 6 Days To Air. The writers room is hilarious.