There are many tools at the disposal of a craft Dungeon Master, and a classic one is clever use of buildup. Over the course of a campaign, there are many things we can build up with locations and people probably being the most common. Who doesn't want to build up their big bad a bit? However, like all things, there are some difficulties that come with the collaborative nature of role-playing games. For that reason, I hope to share some of my experiences. Hopefully it helps someone out there or starts some interesting conversations.
Extent vs. Impact
One of the big conflicts with buildup is how much we build up and the overall impact we get. Doing too much buildup in the wrong kind of way can actually hurt us. At the same time, if we build things up in an exciting way we can still fall into a trap. If we build up a character or location up for the entirety of a campaign and due to luck, creativity or a combination of both the players make short work of it, we created a disappointment. There are some creative and not so creative ways out of this (liches and revenants in particular will be back) but in general it's an issue. There is always the risk of disappointment, it's just the more time and care we spend on buildup the more we highlight one particular thing and increase the risks. If we pull it off, however, we gain. There is also the generally safe method of buildup that leaves things in mystery. Someone sent an assassin and we won. But who? And why? This kind of buildup is less direct and tends to focus on outcomes. It's different than hearing whispers of an incredibly powerful legendary wizard who seems to have returned. Even if they find the source of the assassin and take it out, they still had a combat encounter. If it was a tense one, all the better for the buildup. However, it's important to make sure something comes with this kind of buildup. You'll want to address the source of the assassin at some point unless the players decide to willingly avoid it for some reason.
Events and actions had implications and side effects. These make for great buildup to later parts of a campaign. I mentioned this earlier, but the assassin example is one such case. The way it tends to work is that the players will accumulate questions as they go through their campaign. As they get further along, they'll slowly get answered and lead to more questions before everything, or almost everything, is revealed. Also, as mentioned earlier, there needs to be a reveal. Building up to nothing rarely goes over well. It could even be unexpected, disappointing but hilarious. However, that is still a buildup to a punchline.
Telling and Talking
Hearing things about events that are happening, and certain people can go a ways to building up characters and scenarios. Knowing ahead of time who they just got on the bad side of makes things far more interesting and tense that suddenly making enemies with nameless guy number 505. It's a classic method. “Wait, thee nameless guy 505?” The trick, however, is in doing it in a non-boring way. Players tend not to like their campaign going on pause in order to let the innkeeper go on a long story time that says everything that needs to be said. Instead, it tends to need to be spread out over multiple people and events. At least 3 works fairly well for me. Some repetition can help as well, but it shouldn't be as detailed as the first or it can get grating. I find 3 tends to be the magic number in those cases as well.
Personally, I find that actions and events speak far louder than my characters. It's one thing to hear a story about how powerful a magic weapon is from an innkeeper. It's another to have your players walk across grass turned to stone, hear it crunch as they walk on it, in order to find the magic weapon sitting in the middle of a platform unguarded. It's right there, fit for the taking. It's just that no-one has been stupid enough to move it until now. Everyone in the surrounding area seems to be terrified of it. You can be sure that your players will be expecting something from their new magic weapon. Talking is a way to build things up and yes, technically talking is an action but I find it much better and easier to think of actions in general when I'm thinking about buildup. It's less restrictive that way. It also gives you tools to build up to different things differently. Why should the nearly unknown cult that controls the city be built up in the same way as the puppet leader they put as the head of the city?
Foreshadowing and Uncertainty
There is another factor that makes such things difficult in tabletop games. We can't see the future as Dungeon Masters. However, buildup kind of suggests that we know where things are going. Trying to salvage buildup after thing didn't go according to plan is extremely hard to do and is basically an art. It's because we need to balance not making the buildup worthless at the same time as allowing freedom of choice. In these cases, I also find actions are extremely important. Actions can be interpreted. They don't necessarily have one set meaning. This tends to give more room to be creative than when things were directly told to the player characters. That said, thinking of talking as actions still allows some wiggle room. The people that players talk to could be unreliable or have their own reasons to knowingly lie. This adds an element of interpretation to the buildup and can still make it seem like it was always this way. I would recommend having a light touch for this. It can really make a confrontation more memorable but remembering it, executing it well, and having it be adaptable is tricky.
What To Build Up?
Particularly cool, unique, or important locations tend to be built up. The big villain also needs to have some kind of buildup. You don't want them to come out of nowhere and be meaningless. However, not everything needs to be build up. Having it targeted makes it more effective. Put another way, if everything is being built up nothing is.
Don't Sweat It Since It Comes Naturally
A large part of buildup comes naturally. Players will tend to have multiple run ins with the big bad. These run ins act as buildup. They create a history, build players expectations due to exposure to the character and if done enjoyably, lead to an interesting positive association. They may still hate them and dread seeing them, but also cautiously look forward to future confrontations. Likewise, a super hard to get to location will be hard to get. It may require guide to find, special supplies and a special path. All of this acts as buildup for the location. Trying to shove buildup where it doesn't belong can be one of the worst things you can do as far as buildup is concerned.