Improvisation

Most of the detail in the original Greyhawk Campaign was invented on the spot during play.

From my own experience, I can say that this approach tends to create the most exciting, immersive stories. Some of the best and most memorable adventures with my group were when I just made up stuff in response to their suspicions, actions and to whatever the muse told me. I think this is because when you are not forced to follow and read detailed scripted material, the game flows better, there is no wait times reading up what exactly is in the room, and you are able to focus on whatever the group seems to find engaging most. Of course, once invented, these adventures take on their own reality and become part of the fabric of the world. 

As for making up stuff, that has always been at least 75% of every dungeon adventure I have run that isn't a test of a module meant to be published for public consumption. [11]

I usually made one-line notes for my dungeon encounters, from around 20 to 25 of same for a typical level done on four-lines-to-the inch graph paper--a few more on five-, six, or seldom used 8-line graph paper. the other spaces were empty save for perhaps a few traps or transporter areas and the like.

I did indeed create details for the PC party on the spot, adding whatever seemed appropriate, and as Rob played and learned from me, he did the same, and when we were actively co-DMing we could often create some really exciting material on the spot, if you will.

When the encounter was elimiated I simply drew a line through it, and the place was empty for the foreseeable future. I'd give Rob the details of any session he was not at and vice versa, so we winged all of it. Sometimes a map change and encounter key note of something special in nature was made, but not often. We both remembered things well, Rob very well and when necessary something was made up out of whole cloth for the sake of continuity of adventuring.

When new maps were made it was often nearly impossible to have the stairs and other connections line up with other maps, so a note or two and "fudging" served perfectly well. This was particularly true of the means of entering and exiting lower levels from secret locations surrounding the castle ruins. #4660

Just in case some reader here thinks it odd to create on the fly as Rob and I usuall are want to do:

The main difference between formal creation of material and doing it as one serves in the role of GM is spontaneity, that allowing the material created on the spot to better suit the player group and the situation at hand. Otherwise one must set forth the material to be played and recite it more or less verbatim, forcing the group to its mold. The creative demand is much the same, but the free-style method usually allows for more enjoyment by all participants. I recommend it to all GMs able to manage such playing style #4664

While I do often go with prepared material, especially when I am not feeling really full of energy, the best game sessions are often those in which SOP was in full swing. GMing on the fly is more demanding than is using prepared material, one's own or a published module, but the players generally really get into an extemporaneous adventure, as they are integral to its formation and direction. #4676

SOP, standard operating procedure was to make up stuff "from whole cloth".  

Skim the material, have the map handy, and then, after reading the intro material, some aloud for the players, while remembering the more interesting parts, wing the rest of it. the material is your own when you are running it for your group #6238

Of course most of the long-time DMs will have much experience in winging it even with modules, as that was assumed to be the normal way to run adventures for the first 10 or even 15 years of RPG play.

I suspect that newer GMs are more used to the hand-holding modules, prepare like material for their own campaigns. #4680

The opinions of some folks to the contrary, I have always "winged" most adventures--the exceptions being play-tests of material in a ms. for a module, and a few set-piece places I developed. All the outdoor adventures I ran, and most of the dungeon crawls were half or more made up on the spot. #1706

Most of the time I am GMing something other that a play-test of some module, I wing the whole adventure session, so the palyers are at complete liberty to do whatever their hearts desire. If it is something foolish, I enliven things a bit more than usual...  #6924

I rely on a map for general inspiration, but do not usually have any real notes at all, only pretend ones to make the players think the adventure has been planned. Of course my old-time regulars knew that wasn't the case, especially when they were returning to Greyhawk from the Land of Chin. As well as when dungeoneering, as all of them had seen the encounter notes page for one or more levels, single lines for each of 20 or so places on each level. Dice assisted in that by indicating random encounters, what was encountered...if it fitted. Otherwise I would select the one that seemed right or make up something. #6928

On top of that, EGG and I were both naturally creative individuals whom as co-DMs sought out expansion (and expression) of concepts and ideas as these gathered to us.  These ideas were often spawned during game play, by reading a book the night before an adventure, through scouring our ever-growing notes and sometimes were derived from voiced, but errant, surmises on the players’ parts, such as … “I bet it’s a…”  …or … “It seems like a watcha-ma-jigger” or, more emphatically, “It’s a (fill in the blank for a monster or situation)!” [41]

The level designs and minimal prepared notes provided a framework for the GM to start from. Look at level 1 of the castle for an example. 

Gygax improvised large amounts of content, in response to player actions -- giving them more of what appeard to excite them. He also improvised rules, to keep things moving, rather than looking them up in the book, even where they existed. 

Of course, on the player side of things you would not know what was predefined and what invented, and this approach helped to avoid wasted time looking up what was written in the adventure manuscript and kept the game moving. 

In the original campaign run in Lake Geneva, much of the refereeing was done "seat-of-the-pants" style, and encounter areas were not fleshed out much beyond quickly jotted notes.  [7]

Whatever was there was just to stimulate the DM's imagination and inspire something that suited the party for good or ill. [6]

All the outdoor adventures I ran, and most of the dungeon crawls were half or more made up on the spot. [6 #1706]

Rob and I both DMed on the fly, made only short and often cryptic notes, and thought very much alike, so handing the "castle" back and forth as co-DMs was never a problem. Encounter notes consisting of only one line from which we created reams of information out of whole cloth on the spot. [2]

Rob and I both DMed on the fly, made only short and often cryptic notes, and thought very much alike, so handing the "castle" back and forth as co-DMs was never a problem. The old material would be basically unusable my most others, of course, encounter notes consisting of only one line from which we created reams of information out of whole cloth on the spot; [6 #4414] 

Areas around the city were developed on the spot as the need arose. As a matter of fact, all of the adventures in the City of Greyhawk were “winged”, created from whole cloth on the spot. The many players now active loved this. Even though the castle dungeon levels were deeper, the challenges greater, the rewards richer, proportionally more time was spent in “city adventuring.” [6]

It is so tedious to remember all the minor, but necessary, details. how I do enjoy winging a dungeon crawl with nothing but a map and the sort of encounter noted, creating all the other details on the spot! #2929

In my opinion the best way for a DM to manage monsters is to know what their type is, generaly what an average one of that sort can do, and then...WING IT! #7480

Reacting to Player Action and granting agency

When extemporizing, the GM must be prepared to handle all manner of unexpected actions by the players. If they are foolish, I always invent a number of opportunities for disaster. If the course taken is one that is clever and innovative, I add in rewards.

Before a party goes off on a likley disasterous course I will try to deter them from such action--wandering monsters have much usefulness in this regard.

As for "railroading," there are some scenarios where a bit of that is absolutely necessary to further the whole of the adventure. This is not to say that an entire adventure should be linear and force the party into a situation with a foregone conclusion. The use of a predetermined outcome should be only to set up an interesting and challenging scenario where the players are absolutely free to manage the outcome on their own, that outcome offering penalties for wrong decisions, rewards for correct ones, large rewards for innovation and creative solutions #1706

Incorporating Player Ideas

Gary went beyond just making up missing detail: he was making up the story in response to the players.

Mining the player group for ideas is certainly a good idea, as many heads are often better than one when looking for new and innovative approaches. Of course, that is one of the uses of well-designed modules...fresh ideas slip into one's own campaign. My group always castigates a fellow that mentions something I might do, for they know if it is demanding and I hadn't thought of it, the mention will assure the appearance of whatever was spoken of at some point... [6 #5833]

I would often allow in-game discourse between PCs to develop a plot and direct the general direction of things. Not surprisingly, the players often know pretty well what sort of adventure they would enjoy, so by listening and creating by the seat of one's pants can work wonders...or at least squeek by with something that's mildly entertaining to the group. #287

Of course I am used to winging adventures for a few decades now. It is work when doing so, but it generally is a more enjoyable adventure experience for the players, as one can craft events directly from players' comments and the actions of their PCs. #6916

Outside of the random tables, dungeon components etc. I have prepared over the years, most of which have been published when I wing it I try to suit the adventure to the players and their PCs.

I listen to what the players are saying about the adventure, and take inspiration from chance remarks...as I believe most other GMs are wont to do. #6939

Other than the matter of me doing a good deal of extemperaneous creation in play sessions, I am not a paricularly notable Game Master  [11]

Keeping it up

Gary continued this approach also in later years, during the convention play.

I have indeed DMed a fair number of adventures using other designers' material. The SLAVE PITS series, for one, and another I used often for quick pick-up games was the ABDUCTION OF GOOD KING DESPOT, one of my favorite short scenarios. #1707

Usually, though, players want to adventure in something I have created, so I do a lot of winged adventures outdoors or in the old Castle Greyhawk dungeons, or else use manuscript material I have on hand. To be absolutely truthful, I usually create new material on the spot for my own material that doesn't exactly follow what is written down, as circumstances alter cases :D #1707

Adventure Hooks

I have no standard ones that aren't the usual stock in trade of GMs everywhere. Typically, I inform the team that their renown has reached the ear of so-and-so, thus cuasing that one to want then to do X for him or her. Otherwise, they are assembled because Y threatens them, or they have heard of the fabulous Z and wish to acquire/destroy it. Sometimes, I suggest the adventure is being undertaken because they are bored with the humdrum affairs of the ordinary and seek adventure. Of course, having some friend or relative kidnapped is always a good adventure hook too ;)

Whatever suits the campaign, the players, and the GM's fancy serves as a lead in to the adventure. When I create a module I always try to have the hook included as general as possible so as to be mutable to meet the above. #4415

City Games

A sketch map of the community, a few places of interested highlighted in color, and then wing it. As the PCs develop something interesting by interacting with what I present that becomes a fixture in the town. This might be a particular street thief, a tavern, a stable with a shifty owner, whatever... #8751

[References: see Greyhawk References]