In my last column, Go, Go, Gazette, we discussed the detailed write-up of your campaign "Gazetteer," including your setting map, sandbox key, and story web. If you followed those guidelines, you've ended up with a gazetteer with about 40-50 static points of interest, including settlements, lairs, and dungeons, spread across a campaign map; and about 40-50 dynamic lairs to use for random encounters in empty areas.
At this point you have what my friend Zak Smith, the genius behind I Hit It With My Axe, calls an extreme sandbox, in which the PCs can go anywhere and do anything. The world is essentially a very large dungeon and the points of interest are set-piece encounters or traps. The clues in your story web are like corridors that carry your party from one adventure to the next. If you just want to start running your campaign, you can actually stop world-building right here - it's a perfectly serviceable way to play. But if you want to engross yourself further, the next step is to put your world in motion.
A world in motion is one in which effects occur of which the players are not the purposeful or apparent cause. I say "purposeful or apparent" cause because, in many cases, the actions of the players will actually be the cause, through triggering, but this might be without their knowledge or intent. There are four basic techniques you can use to put your world in motion: triggered events, wandering NPCs, hand-crafted content, and random events.
Sometimes You Find Trouble...
A trigger is an event that precipitates other events in your campaign world. Triggers are exceptionally common in videogames, and it's a safe bet that anyone who has played a computer RPG is quite familiar with them, so I won't belabor you with too much definition.
The key value of triggers is that they allow you to offer content that is time-sensitive. For instance, gladiatorial games are very interesting while they are occurring, much less so after they've occurred. One could include gladiatorial games with a trigger: "The first time the party visits the Imperial Capital, the town criers are announcing that the Great Games are to be held in three day's time. Gladiators are needed!"
A more advanced use of triggers is to have a trigger in one location set off results in other locations. For example, "The first time the party visits any village after the Summer Solstice they see proclamations announcing there will be Gladiatorial Games in the Imperial Capital in a fortnight."
Triggers can also be keyed against each other. Imagine that you place ancient calendar stones hidden in the wilderness that warn of the end times. At whatever point the party discovers the first calendar stone, the end times are set to 2 years away. If they later discover other calendar stones, they adjust based on what was first triggered, so that if it's been ten months since the first calendar stone was found, now the end times are fourteen months away. Other triggers placed within the setting can then be keyed to whether the end times are near, with encounters that change to include additional undead, more powerful magic, and so on.
The risk with using triggers is that sometimes the party can set off triggers on a timeline that is quite different than what you envisioned when you put the trigger in place. For instance, imagine that they stop into a village to have a companion magically healed, and sets off the "gladiator games" trigger. It's highly unlikely the party will stop its quest to go to the capital on short notice; if you trigger the event, it's going to be wasted. What to do in such a circumstance depends on your free time and your personal judgment. If you have sufficient time and energy, you can allow the event to be "wasted" and write up what happened in the party's absence; this will let them know that the world really does go on without them, and that there is an opportunity cost to each decision. Alternatively you can delay the trigger until they've finished the quest, or modify it on the fly to extend the time available to give them a real chance to follow up.