Originally, Matt Damon (who Trey Parker and Matt Stone have admitted is really a "pretty cool guy") was going to be portrayed as intelligent and articulate, but when they saw the puppet, they noted that it made him "look retarded" and decided to portray him as such.
Every time I think of that, I crack up. And the "America, F* Yeah!" song is still running through my head.
I can't get the "montage" song out of my head. "Even Rocky had a MONTAGE..."