That is just one example. I have definitely prepped material, whether it be NPCs, places, etc that never come up in play. That is just part of running a sandbox, you make stuff knowing it might never come up.
I have found myself prepping less and less of late...not sure when that started.
I have set scenes which we roleplay through, I'm not sure if they should really be called framing. I do not believe they are in any way the type of framing
@pemerton produces.
So generally I have a few ideas of scenes to run through or 1-2 combat encounters and then the rest is created on the spot.
As an example (sorry it is a little long) - I knew I wanted to run a situation where a cloud giant fortress stopped over Waterdeep as per the entry in the Storm King's Thunder AP. I knew that since the PCs were the Council's representatives they would send them up via griffons to meet with the giants and enquire as to reasons for their arrival.
So besides the entry in the book about (i) them wishing to meet with city officials who could speak to the history of Waterdeep (as the giants believed the city was built on some giant ruins which they wished to investigate old city maps etc), that (ii) the Countess distrusted small folk and (iii) implied that the giants were the isolationist type, the rest of the entry was bare bones. I had NO plan.
Two of the four PCs went up and met with the Count and Countess. So after I provided a montage of imagery of the journey and fortress on the cloud, it was the conversation that arose that set the ball-rolling.
One of the PCs began informing the cloud giants about the recent ongoings, the rise of Heartkiller (Adventure League SKT), their rescue of King Hekaton and that he was off to save his daughter. It quickly became apparent to the PCs that these giants were not aware of the recent events and seemed content to stay out of it. I still had NO plan.
Then after the PCs learned the reason for the giants' visit, the same PC offered to have them talk to King Hekaton as as he explained, they had direct communication and they were going to meet with the King to help save his daughter. He had not told as yet told the Count that he had a
Sending Stone in his possession.
THAT is when the plan materialised in my mind. These giants were isolationists not partaking in the politicking. They did not want to choose sides, they did not want others to know they did not want to choose sides for who knew who would eventually rise to power - King Hekaton or Heartkiller?
These small folk (the PCs) were now complicating matters for them with their direct line to King Hekaton. Surely since they were so intimate they would reveal the cloud giants' reluctance to partake in the power struggle between Hekaton & Heartkiller.
At that point some knowing looks passed between Count and Countess and the Count requested to speak with the King, his intention being to destroy the means by which the PCs communicated with the King (I had envisioned he would have crushed the
Sending Stone). The other PC noticed the strange ongoings (the player was also smart to all the clues I was providing) and immediately lied saying the ability to communicate with the King was down in Waterdeep. He didn't know exactly what the Count intended, he only knew that the strangeness of the situation prompted a lie.
A opposed Deception vs Insight was rolled, the PC used a fighters maneuver (d10) and pushed his score to 30. He lied so well they believed him.
And with that the Countess gave another look to her husband who in turn said that he would need to discuss their proposal about joining the King to his subordinates and that they should accept the giant's hospitality by waiting for some time, possibly staying the night while they deliberated. They were also distinctly aware that the fortress was moving (away from Waterdeep).
Not wanting to cause a scene, but taking care of the exits (1000+ feet in the air, their griffons stabled and under guard) they were escorted to a room and had their door locked. Now the PCs had no intention of staying the night, the campaign is quite time sensitive and they couldn't afford this distraction and certainly did not wish any conflict.
Food and refreshments were brought to them as they pondered if they should just escape or wait it out and see how it plays.
At this point, I wasn't sure how it was going to play out, only in that the giants, as presented, were not necessarily killers.
2-3 hours of in-game fiction passed, and the players were anxious (as was I

). They were summoned back to the meeting chamber where they were asked a series of questions and whereby the giants also disclosed their fears. The PCs answered honestly and I was running a variation of a
Zone of Truth cast by the Countess so that only once a PC lied would there be a need for a Charisma roll, so there was no indication given of the spell being actively present.
Having passed their test, the giants conceded to letting them go reluctantly having to trust that the small folk would not betray them, and also not wishing to harm or inconvenience the Champions of the Council. So the fortress sailed back to Waterdeep, the PCs flew back home, with the cost of the full evening now wasted.
And the giants abandoned their research and left the skies of Waterdeep.
There were other related incidents but yeah this was all born by me (fully GM decides) on a nothing write-up of the giants in the book and the result of 1 contested die roll (Deception vs Insight).
All this to say that this little
adventure was born from almost no prep.
None of us deny the power the Gm has, but the GM is not expected to just declare everything and anything, and the players can act on the setting through their characters: which matters a great deal in shaping how things go
That is certainly true based on the above.
If the PCs acted out aggressively or tried to make an escape or were caught lying...I'd have to think how it may play out and what the possible consequences would be.
To be honest, I would feel comfortable consulting the players for ideas, make a quick table and have us roll for it.