Encounter building is more than just combat. In effect, every aspect of your session that drives things forward is an encounter of one sort or another. A trap is an encounter. A conversation with a key NPC is an encounter. Knowing the skeleton of a good encounter is a core part of building the scenes and scenarios that fill up a session, and thus a campaign. So let's take a look shall we.
Three Key Questions
In my experience, and research, when looking at the design for a fun encounter you can break it down and answer three key questions about it. Since we can take a functioning, good encounter and do that it stands to reason that by tackling these questions one at a time we can then build a good encounter.
What are the questions?
- What Question Is This Encounter Answering (a.k.a. What is the Dramatic Question for this scene.)
- What is the source of conflict in the scene?
- What are some approaches that could be taken in the encounter?
What Question Is The Encounter Answering / What Is The Dramatic Question For The Scene
This is the goal of the scene. A character or group of characters is trying to answer a question of some sort, and so we end up with the encounter. This is effectively the sought after prize. "Will Inigo Montoya be able to get revenge on Count Rugen?" is the 'Dramatic Question' we are answering in Inigo's final showdown with the Count in the Princess Bride. The end of Star Wars: A New Hope has the Dramatic Question of "Will the rebels be able to blow up the Death Star before their base is obliterated?"
While you can pose a dramatic question for a scene from either side of the conflict, for RPGs it is best to just focus on the perspective of the PCs as that is going to frame the game.
Also worth noting is that a single dramatic question can be drawn out across multiple scenes and encounters but each one should teach us more about the answer. For example, Wesley - known then as the masked man in black - pursuing Inigo, Fezzik, and Vizini begins with the realization of a ship pursuing the villainous (at the time) trio and doesn't end until a solo encounter with all three. The question being "Who is this masked man, and what does he want?" Each encounter then teaches us more about him. From Inigo we know he is skilled but courteous. From Fezzik we learn he is strong, but gentle. From Vizzini we learn he is clever and shrewd. Finally with Buttercup, the end of the encounter, we learn who he is and why all of this is happening. But note how the question is "who is this person?" and not "can this person get past obstacle x." While it is not wrong to have a Dramatic Question with a yes/no answer, especially near the end of a story, having questions with more open answers gives a lot more freedom.
What Is The Source of Conflict
If you don't have conflict there is no need for it to be a scene. This is why so many shopping trips and little mundanities in games can and are solved with a hand wave. "Can I buy a sword? Sure, 20gp." There's no need for a scene, because there is no conflict. With an encounter, there should always be a conflict. The trick is that while the conflict itself does not need to answer the Dramatic Question, it does need to illuminate the answer to it.
So how do we get conflict? Simple, we have goals that are opposed to each other.
Group A wants something. Group B wants something. A and B can't both have their thing at the same time.
The Empire wants to snuff out the Rebellion. The Rebellion wants to overthrow the Empire. That is the conflict in the background for the original trilogy of Star Wars movies. In Empire Strikes Back Han wants to be with Leia, who for her own reasons does not want to be with him despite liking him. This conflict fuels a number of scenes between them, culminating with the "I love you." "I know." exchange at the end of the movie.
What do your PCs want? What do the NPCs want? Why can't they both have their way? This is what brings things into conflict.
This is also how you set the goals for each side in an encounter. Hungry bandits looking for food and coin (to buy food) vs. PCs rich with loot and supplies traveling back to town. The bandits want what the PCs have. The PCs want to keep their treasure from the recent adventure. There is conflict.
The King's advisor wants credit for presenting a solution to a problem to the king. The PCs want to inform the King of something going on as fast as possible. If the PCs get straight to the king, the advisor can't use it to curry favor. There is conflict here, even though it is not the type that should result in weapons drawn and combat initiative.
What Are Some Viable Approaches?
This is a very different question than "what is the solution?" In general I frown on making solutions to puzzles because when you make a solution your brain is less open to other possible solutions because - to you - there is a right answer they have to find.
However, taking a few moments to sketch out some possible approaches can help you be prepared for how your players tackle the encounter. As a rule of thumb, expect combat/violence/threats of violence to always be on the table. There's a reason RPGs are full of stories of players going to buy some gear and coming out with the city on fire and their faces now on the Top 10 Most Wanted list. It's not every group. It's not every situation. But you never know when a player is going to decide violence is t he most expedient route and warranted here, so just be ready.
In general I like to think of what would happen should the PCs try a social approach, a commerce approach, and a combat approach. Obviously certain encounters lend themselves better to certain approachs (Social or Commerce will work better on the above mentioned king's advisor for example, but combat may still work.)
This prep can be as easy as "What happens if the PCs try to talk?" "What happens if the PCs try to bribe?" and "What happens if the PCs respond with violence?"
For example, the bandits raiding the PCs camp at night is a likely combat encounter. But what if the person on watch notices the approaching bandits and invites them to the fire to share what's left of dinner? The bandits are hungry, now they've lost surprise, but food is offered. Do they go for it? Do they attack? Do they retreat?
What if the PCs offer them coin to just go away? Do they take it? Does it embolden them to attack?
Finally, what if the PCs attack? At what point do the bandits flee? At what point do they let the PCs flee? Do they kill if they end up ahead? Do they try to wipe out the group or just to distract while some steal food/money and run?
Bonus Question: How Is This Going To Be Fun And For Who?
If you have the answer to the above three, it can also be good to ask yourself how the encounter will be fun and for who. Big encounters should have something for everyone ideally, but smaller encounters may be designed to showcase certain character strengths and skillsets.
Just remember, it's a game. It is supposed to be fun. For them and for you.