Personally I tend to set up the "rules" for each encounter differently depending on the context and what I think the PC's are likely to do.
So, for example, in a recent encounter I had 3 goblin sharpshooters riding 3 gray wolves, chasing the party.
When working through my "Tactics" for the encounter, I included a paragraph like:
The group comes in wolf/partner pairs. The wolf will fight to the death as long as their partner lives, but will flee if their partner is killed and the wolf is bloodied. The shortbowmen will call their wolf to them to mount their wolf and flee if both partners are bloodied, or will flee if their wolf is dead and they are in imminent danger of being brought into melee combat.
With the classic "skirmishers in melee, archers firing down upon them", it worked out very well, with the party fighting the wolves, then the archers turning to run (some escaped, some were hewn down) once the party had worked their way through the wolves.
In another encounter, I had a repeat named villain, Ashtuk, who had been hunting the party down, and challenged the tripleheaded-flails-ranger to single combat .. with enough of Ashtuk's friends hanging out to make it a fun challenge whether the party let the ranger fight honorably, or tried to manipulate the outcome.
I had the following written:
Preparations: There is no question of Surprise. Ashtuk waits for Spike’s response.
Insight – DC10: He seems to be inviting Spike to a one-on-one combat – though his friends look like they’d be more than happy to fight the rest of you.
Insight – DC20: You do think he means it to be a fair, one-on-one fight. Fresh, they might have been a match for each other, but you doubt Spike is in shape to take him alone.
Perception – DC 15: Two of the other orcs are wearing scale mail, carrying shields and a bastard sword. The other six have simple hide armor and clubs.
Intimidate – Ashtuk does not intimidate.
Intimidate - DC 21+: Scares off ROLL-20 minions. E.g., a roll of 24 scares off 4 of the 6 minions.
Diplomacy – DC 10: What do you want?
“Now? To kill you, three-flails. Then, we take the girl.”
Diplomacy – DC 20: to substitute another opponent for Spike.
As it turned out, after the DC10 Diplomacy success, the 'girl' in question decided to ask "Wait, why? What do you want with me" .. and then nailed a 29 on her ensuing Diplomacy check, which I hadn't considered but a natural 20 will do that to ya! I decided that she'd surprised Ashtuk into a bit of overconfidence, and he made the classic villain mistake of telling them his plan. In this case, telling them WHY he wanted the girl ... which led to the party negotiating .. though negotiations broke down when they discovered that his "best offer" was "I'll give you the 60 hostages if you give me the sorceress."
. . .
(The ranger took Ashtuk on in single combat, while the orcs taunted the rest of the party for not fighting them .. when Ashtuk got the upper hand, the bard snuck him a healing surge, but got caught out the second time he tried it .. leading to a battle between the other eight orcs and the rest of the party, both sides dancing around the single-combat and not engaging either combatant .. which conveniently wrapped up right as the ranger got a badly wounded Ashtuk to surrender .. great fun had by all!)
. . .
So, for me its about anticipating which things the party are likely to attempt, and setting appropriate DC's based on the direction I want the plot to go ... I do think of the NPC's as "characters" and try to have their motivations built out well in advance of the party running into them, so I don't want to provide an "open" system in which the party could, conceivably, have befriended Ashtuk with that shock 29 Diplomacy role.
