On freeze-frame vs timelines (which [MENTION=6775031]Saelorn[/MENTION] and [MENTION=82106]AbdulAlhazred[/MENTION] have been discussing above) vs simple setting description, consider the following three instances of GM prep.
The GM preps setting
The GM's setting notes say "On a typical morning at the Garden Gate of the city, wagons, visitors, the odd mercenary etc line up to enter the city. . . [goes on with more detailed description].
The GM preps timeline
The GM's timeline sets out a projected sequence of events in the absence of player interference via their PCs: On day 1 the weapons are smuggled into the city via the Garden Gate; on day 2 the insurrection breaks out; on day 3 the baron's militia takes control of the neighbouring villages; and so on for as many days as the GM thinks are worthwhile to plan for.
The GM preps freeze-frame
The GM's notes say "When/if the PCs come to the Garden Gate, they see an accident - a wagon has tipped over, and the hay spilling out of it has revealed unlawful weapons underneath. Half-a-dozen guards are coming out of the barbican gates as the driver of the wagon tries to blend into the crowd."
Now, consider an episode of play in which the PCs approach the city in question for the first time, travelling along the road that leads to the Garden Gate.
The first GM describes the scene, reading from his/her setting notes: "You see wagons, visitors, the odd mercenary etc lining up to enter the city . . ."
The second GM looks up the entry on the timeline relevant to the current day of play. So if it's day 1, the GM describes the scene much as the first GM would, though with a mental note that if any PC says they're searching a wagon there is a 50% chance they'll see weapons under the hay. If it's day 3, the GM describes the smoke coming from inside the walls of the city, the baron's militia blockading the Garden Gate from the outside, etc.
The third GM reads from his/her freeze-frame description: "As you approach the gate, you see that a wagon has tipped over on the rough road. There's some sort of commotion, and as you look again you can see why: the hay spilling out of the wagon has revealed weapons hidden underneath it! Half-a-dozen guards are coming out of the barbican gates as you see the driver of the wagon try to run into the crowd. What do you do?"
How is one of these more of a railroad than the other? In each case, the GM is framing the PCs into some fiction. No outcome has been determined. On the face of it, none of them looks like a railroad to me.
Is any of these more conducive to player agency than the other? In the abstract that is hard to answer, but I think everything else being equal perhaps Day 3 of the timeline, or the freeze-frame, is more likely to engage the players and lead to some interesting action declarations than is the description read by the first GM, or Day 1 as read by the second GM.
Is any of them more or less verisimilitudinous? On the face of things, not that I can see.
What is the difference between them, in terms of prep and play experience? To me, it looks like GM 1 has written a boring freeze-frame, and GM 2 is going to give you that boring freeze-frame, or a more interesting one, depending on what day it is on the campaign calendar. GM 3 is going to give you an interesting freeze-frame no matter what.
That is why I said, upthread, the difference between a freeze-frame room and a non-freeze frame room isn't that one has an event occurring and the other doesn't, but rather is that one has an
engaging event occurring that probably doesn't extend over a long period (and hence is perhaps more coincidental to stumble upon) whereas the other has a more mundane event occurring that probably extends over a longer period.
To give another example, compare: "the goblins are sitting in the guardroom playing dice" - not freeze-frame - with "the goblins have been playing dice in the guardroom, and a fight has just broken out over who is entitled to the stakes" - freeze-frame. But the difference between the two isn't that something is happening in one but not the other, but that something relatively mundane and extended is happening in one, whereas something interesting and immediate is happening in the other.
My preference as GM is to err on the side of interesting. That has no connection to railroading.