One moment, you're neutralizing a pawn to complete a mission objective and capture points. The next, you're a vile murderer ambushing a penniless grunt to get your hands on the castle's loot. What happened?
The guard opened his mouth, that's what happened. You're playing the first of level of the "immersive sim," Thief: The Dark Project, in 1998, when the polygonal soldier whose gullet you were about to lance with an arrow mumbles to himself on his patrol.
GUARD: Everyone above me gets all the favors and I haven't had a thing to eat in days.
Words change everything. Like dousing or lighting a torch changes the nature of the environment in Thief, hearing, reading or missing a line of text changes the intellectual landscape for the player. Consider how this note, pinned to the kitchen wall in that castle, changes the way you render the environment in your imagination:
Please speak to Cook about last night's dinner. While, technically, the menu conformed to my instructions, I suspect that the lamb was somewhat older than this spring's, and I am in no way fooled by his practice of warming the salad to disguise wilting. If Cook is incapable of finding adequate ingredients, he can be replaced.
- Lord Bafford"
By itself, it's just a simple tool to evoke an opinion about the absent lord whose stuff you're stealing. (Probably, you get more satisfaction out of robbing a whining schmuck.) But, if you happened to overhear that first guard's mumbling, the words amplify each other. Now, Lord Bafford is complaining about the technical conformity of his meals while his soldiers are going hungry.
Are you going to try harder to boost every scrap of his loot, 'cause that'll show him? Or will that just get his hungry guards punished for incompetence? What's going to happen in the castle after you're gone?
It doesn't matter. Nothing happens after you leave the castle - when you're finished, it ceases to exist. What matters is you bought into it implicitly for a moment or a minute or 10 minutes, while you played. You enjoyed the illusion. So, it does matter.
GUARD: What is that smell? Smells like... old meat.
The devil's in the details. One line of dialogue makes a room smell like rot. One note conjures a person out of nothing. In Bafford's castle, his journals show he suspects someone called Ginny is stealing from him, and he's trying to dig up dirt on "Viktoria." In your imaginarily rendered game world, these people exist out in the city somewhere now, but you don't know who's just background or who might step into play. Anyone could be Orson Welles' Harry Lime.
This background isn't just color, it's vital for creating a living environment, and that's vital for stealth games. For the player to feel like an intruder or a sneak, the environment can't seem to be waiting on him. Guards must seem oblivious to the player's presence (even though they exist solely for him). So, overheard conversations and peeked-at notes may be unconnected to the game's story and inconsequential to the successful completion of the level, but they're essential to the voyeuristic atmosphere that defines stealth gameplay. For a stealth-minded player, each uninterrupted conversation is a reward for quality quiet - proof that she's a masterful sneak.
A living background creates an illusion of agency for both the player and the environment. When Doug Church, a designer on the original Thief, talked about agency at Gamasutra, he was speaking mostly about how players need to feel their actions have meaningful effects on the game world. "Agency is more important for playfulness than entertainment," said Church.