Managing expectations is one of the major responsibilities for a Dungeon Master. An area where this is incredibly important is when dealing with combat. Player need to know what they are getting into. To make things more difficult, things often change. The high risk combat situation from last session's dungeon delve changes into light risk combat and mostly role-play this session. Handling this change is an art, and in hopes of helping others as well as myself, I'll explore my thoughts on this subject.
Setting the Basis
When a game starts, there are some big overarching things that need to be set. A big one is the deadliness to expect. And again, I'm not talking about if the players have a chance for their characters to die. Some groups don't like running. They want the encounters balanced so that they have a good chance of vanquishing their enemies. Others are fine when a victory in an encounter means getting away alive. However, if they know from the get go that they may need to run, that idea sticks. They won't always run, but they'll know it's a valuable tool in their arsenal. If you instead build the idea that they can win any encounter with proper planning, they may stay and fight even when it doesn't make sense. Adult dragon at level 5? Well, our Dungeon Master wouldn't use something we can't kill. The towns folk telling us that we aren't strong enough is just to build tension.
You can also do this with words from the very beginning during your session 0. It's the safest way. More experienced players will know roughly what it means to face a wight or vampire spawn at level 1, however existing players won't necessarily know they should run.
Re-Establishing the Situation
I find it's a good idea to have a couple of hints from the beginning for players to know what to expect. There are many techniques that can be used for this. If we are talking about a dungeon, the general expectation is that it will get harder the deeper they go in. If the first room is a tough encounter, the players will be weary. Bodies are also a good signal of things to come. If there is a fresh body torn in half, bonus points if it's someone they met earlier so they roughly know their strength, it tells players to be on their guards. Footprints, movement reports from scouts in the area, patrols being decimated, and other battle scenes also help set up player expectations of what they are wading into. It's also a great opportunity to help develop the story as well. If the party they previously helped is found in a zombie state, they'll be concerned and also start thinking of the possible reasons. Necromancer? Wight?
Be Weary Of Tweaking Creatures
Some vampires are stronger than others. It makes sense that some individuals will be exceptions to the rule. However, we also need to be careful when making alternate versions of a creature to put against our players. The first encounter with a creature will set their expectations for the ones that follow. Again, this is especially true for new players but also to a degree for ones that are experienced. Even if the Monster Manual gives a general range, that doesn't mean vampires in your world will work the same way. Vampires might be beastly in appearance instead of humanoid. Or this one might have access to items. Or have a permanent injury inflicted by the arch mage the players met. In these cases it's a good idea to mention that the creature is an exception, preferably in world. Have one of their wizard contacts mention how it must have been a weak variant, or they'd have been torn to shreds. Or how they got lucky and managed to find the vampire while it was resting during the day. Some rule systems have multiple variants of the same creature so players know what to expect, or at least should be aware of this practice out of the gate.
There is a tug and pull between wanting to continue for the day and resting to regain limited resources. If there is no pressure, the answer is simple. However, having some knowledge about what's going on goes a long way in allowing players to make decisions. If things are quiet, you can expect more use of utility spells and being right on the heels of the criminal. If they are fighting a vampire spawn who knows they are present and they are lower level, they'll probably be more cautious. Of course, they can retreat if they run into a vampire spawn while out of resources. If they missed the clues, it may be their only choice. The act of making the decision can often be a source of the fun, as can piecing together the clues.You also want to have some pressure so the players don't rest every 10 minutes of play. Maybe that vampire spawn will get away if they don't chase it now.