Cast at the top of their game.
This movie was one of the funniest I have seen in a long time. Ferrell and Reilly are up on their game, and its obvious that they had a damn good time making this movie. Before seeing it I remember seeing some behind the scenes stuff on the film and each of the cast members were talking about how they were practically given free reign over their characters. They were given the chance to improve so many different versions of each scene just so they could get the the best / funniest variant of each take. From what I know they not only were allowed to throw in various improv lines but they also added to each others lines by coaxing each other into trying something different.
The movie was nothing short of hilarious.