On the GM's side of the table, we have the dissociative game. This game includes the creation of background materials - including composition and understanding of the game milieu - as well as running the game, knowing and interpreting the rules, and taking the roles of any NPC (including monsters and deities as well as sentient human[oid] types the PCs might encounter).
Even in cases where the players share in the dissociated game of building the campaign milieu on the large scale, there will always be information disparity between the players and the GM. Those of you following the comments in this post series will be aware that there has been some contention over how much the players' involvement in creating the game milieu limits associated game play, and I will address that very question in part V of this series.
The same commenter notes that he does very little prep, and largely makes up the game milieu as he goes along. Well, that is obviously one way to deal with milieu creation, and I dare say that no matter how well you prep, there are going to be times when the direction chosen by the players force you to make up material "on the fly" to some degree or another. Some people are very good at this; they need do little or no prep work because they are capable of astounding works of unique genius at the drop of a die. I certainly cannot work like that, nor have I ever seen it in action. That doesn't mean that it doesn't exist, just that I can't recommend that as a working model.
When a television series is created, there is often a "series bible" that helps to maintain continuity within the series. Rather than making up the milieu episode-by-episode, the series creators take the time out to flesh out the story arcs and milieu in which the episodes will take place. When this is not done, or is not done well, it can be irritating to the viewers. Surely I am not alone in wondering how Sheriff Rick can stumble on an unexpected prison within a couple of hour's drive from his house in The Walking Dead? Better care in the creation and presentation of the milieu prevents mistakes like this from occurring.
Earlier I had said that the prospective GM must be able to view any given portion of the game dispassionately. I still hold that to be true. The GM can and should be excited about running the game, but he cannot be so invested in a location, encounter, trap, monster, or NPC that they become more important than the players' ability to alter, eliminate, ignore, or avoid them. Simply put, while the GM must decide what, say, an ogre will do to avenge its slain pet owlbear, or what Captain Midnight will do when the PC superheroes go rogue, or where the Venusian Pirates are to be found, he must not advocate for any of these elements in the same way that the players advocate for their PCs.
One hears horror stories of "DM PCs" where the GM actually does advocate for an NPC in the same way that a player would for a PC, and the GM's information disparity allows these NPCs to be more effective than the PCs in every way possible. Sometimes the term is used for an NPC that has a long standing in the campaign milieu, but again the GM must resist the temptation to play the associative game with these characters.
In a true "GM vs. players" game, the GM cannot lose. He controls the pieces. He controls the rules. He can create new pieces at any time, and can replace die rolls with fiat. What an utterly boring and contemptible game that would be!
This is not to say that the dissociative game sucks - far from it! For some of us, this is the game that is most interesting. It offers creativity that even the most amazing associative play cannot rival. It is more challenging. It requires self-discipline. It is a joy to do well.