The problem with arranging Matt Stone and Trey Parker's guest appearance at NYU's Tisch School of the Arts was in keeping it a secret from students beforehand, the professor whose class they visited said on Thursday. "The producers warned me just keep on your toes and be prepared for anything," professor Ken Liotti said via telephone. "This was all spontaneous stuff. They were either going to come into the class 10 minutes in or an hour in." The South Park and Book of Mormon creators worked out a "shave-and-a-haircut" knock with the professor to identify themselves, "so in case some student knocked I wouldn’t accidentally introduce Trey and Matt." 

Stone and Parker dropped in Liotti's Storytelling Strategies class for the show "Stand In," on MTV's college network, mtvU, in which celebrities make surprise appearances at college lectures in their field. In a video clip The New York Times ran on Thursday, Stone and Parker explain how to get through a story by linking scenes with "therefore" and "but" rather than "and then." (It's worth watching.) Liotti said they also talked about procrastination, the bane of almost every writer. "They kind of just said you just do it. You procrastinate, you know you’re procrastinating, and then you wait for the last minute, and then when they’re forced to do it, then they do it. Procrastination is a part of the process. I spoke about how when I write I start to realphabetize my album collection. And they said yeah, we do that all the time, but when the deadline approaches and you have to do it, you just do it."

It was important to keep the writers' appearance a secret, said Richard Pierce, deputy director for public relations at Tisch. "The big thing for [the producers], as I understand, was that they wanted the reactions on those kids faces when they walked in the room. And they got it." In the video clip, students mouths open wide with surprise and delight. It was their first day at school and the first meeting of Liotti's class, Pierce pointed out. "They were worrying anyway, wondering what this class was going to be like. There’s a lot of anxiety," he said. 

The show producers and NYU staffers snuck Parker and Stone in through the back door of the lecture building and kept them hidden before the class started. "We do get a fair amount of well-known filmmakers, actors, media types visiting from time to time, so the kids are used to that to some degree," Pierce said. But "we didn't want anybody to tweet Trey Parker and Matt Stone are in the building." Liotti said he had a hard time keeping the cameo under wraps before class. "The guy was setting up, and I had to tell the PAs to hide the cue cards because they had Matt and Trey’s names all over them. There's camera guys all over the place... but it went off great."