PC Gamer has put together a small oral history of sorts about what they -- and probably many of us -- consider the best Psychonauts level, "The Milkman Conspiracy." A quote:
That was only possible because writing all the dialogue came last. After the designers and gameplay programmers had finished, Schafer would assess every piece of the level, and write dialogue based on all the work that came before. "That was the most solid foundation for the jokes to get layered on top," Robson says. "Half of my memory of Milkman is playing it without any of that dialogue, so that stuff still almost feels like a sort of recent edition. And then after you're done with the level, six or eight weeks later, this dialogue appears all of a sudden in the game."
Now run and read it.