Continuing the discussion from Playing to the audience when the party is split:
I have two somewhat different modes that I use for short-form games (in which I am under pressure to achieve a satisfactory effect in a single session for a con or an ad hoc scratch game) and for games in a longer or open form (when I have time and room for digressions and explorations and more trial and errors, and when an incomplete resolution of conflict can be treated as the set-up for a next act).
The short-form stuff involves generally higher stakes, tighter crucibles, clearer choices between distinct alternative courses of PC action, and a more rapid escalation to violence. In short-form games I more often have a preconceived idea for the story, or a little menu of possibilities, and I nudge the course of events towards an opportunity to work my spectacular effect, take my bow, and ring down the curtain. That material tends to be a bit more railroady, and to feel more like an entertainment that I have provided than a game I have played. But players tend to like them a lot and remember them as tightly-plotted and exciting collaborative stories with conclusions that were unexpected but seen in retrospect to have followed naturally from everything that went before.
With more time and a wider canvas I like to improvise and collaborate more profoundly with plot. In a typical adventure I locate the PCs in their status quo, and then present an initiating incident. That draws the PCs into a developing situation with a core conflict. Character responds to incident. Incident arises out of character as the PCs and other characters react to changes in their situation and thus cause changes to the situation of their counter-parties. There is some sort of crucible in place that prevents the PCs and their antagonists from avoiding the conflict by fleeing or retreating into passivity — very often I rely on the PCs’ duty or business interest, or their steadfastness to a core motivation to provide half of the crucible². In the course of an escalating conflict, the PCs find themselves facing obstacles and lacking information. They must investigate, explore, do things to elicit informative responses, make and execute plans, act on contingencies if they are to respond to incident in a way that with further their goals. This will involve them with more NPCs and locations, subplots and subquests and subsidiary conflicts, and will gradually both reveal more and more about the situation and also raise the level of conflict with the primary antagonist. Eventually, either the players will come to a key realisation and solve the mystery, or else the level of conflict will rise to the point where one party or other makes a fateful decision, and then there is either a revelation or a crisis that resolves the conflict and allows a denouement.
My stock in trade in such GMing is intricate but consistent world-building. In my games the players can be confident that none of the scenery is painted flats, that everything will respond realistically to anything that a character might do to it, and that they can use anything they find for anything they can think of. The danger of that is that there is no law of conservation of detail³; many things have intriguing idiosyncrasies that could perhaps be used, but that are not themselves relevant to the PCs’ interests or goals. These can easily lead the PCs down rabbit-holes. I don’t care to play both sides of the table, so I seldom tell players what their characters ought to do. And I don’t run a Joseph-Campbell world that will keep sending the PCs repeated calls to adventure until they deign to respond. So I need to players to be active and purposeful. Any character can do anything, but I require willing co-operation to the extent that they will do things that respond to the current conflict. It turns out that I need to negotiate explicit buy-in to get that.
I run — used to run — a lot of campaigns in which the PCs are a professional team with shared duties, responsibilities, or business, and in which the adventures consist of or arise out of their going about their duties or business. For example, I have run a lot of adventures in which the PCs were a team of detectives with mutually supporting schticks whose adventures are their investigations of crimes in their jurisdiction and their resolution of the conflicts arising. I have run a lot of adventures for teams of undercover operators whose adventures were the clandestine operations that their bosses assigned them to. I have run campaigns in which the PCs were a small firm of private investigators hired by more-or-less mendacious clients¹ to poke into murky and usually criminal affairs. I have run a lot of fantasy in which the PCs were a clique of inseparable friends among the retainers of lord with a complicated dynastic situation and a difficult administrative duties, or a clique of inseparable friends among the residents of a remote mountain monastery that devotes mystical powers to hunting. I have run campaigns of Saturday-matinée cliffhangers and weird tales for cliques of inseparable friends in the faculty of Walpurgis University. I have run a campaign for a duo of unscrupulous archaeologists/art thieves/con men. These party concepts supply the players with a built-in motive to respond to a recognisable sort of initiating incident, a menu of responses, and a motive to persist in spite of conflict. Knowing what these are I can design a situation that will turn into an adventure for those characters. And knowing what these are going to be the players can design characters who won’t ignore or break those adventures.
¹ Though I have never got around to running the adventure in which a muddy black Labrador leads an under-age blonde into the PIs’ office and then runs off to fetch a bundle of slobbery hundreds.
² That’s where the stipulation of ethos comes in: e.g. “the PCs are a team of ILEA detectives who investigate crimes and bring criminals to justice on exotic worlds in Flat Black”. Unless the players provide me with that much co-operation I find it tedious, time-consuming, and eventually difficult to construct a new crucible for each adventure. Without a crucible the answer to “what would <my character> actually do?” is too often “avoid this dangerous and stressful conflict; sit this one out”. And then it would become tedious and difficult for the players to keep their characters involved.
³ Or rather, I don’t provide the conservation of detail. Players who want conservation of detail have to avoid investigating irrelevant trivialities, especially when the party is split.