In Thief, the illusions of agency enable play. It's possible to complete the Bafford castle level without stealth, killing every bloke in the joint and running like a maniac through the halls. It's undeniably entertaining, but what makes stealth play exciting is the expectation that consequences exist for such behavior, even though no one can actually step out of the background to punish you. The next level begins how it begins, regardless. Words making you think the background environment is more powerful than it is fool you into thinking you're susceptible to forces that have no actual agency over you. The sim has immersed you.
The illusion has a real voice. When words change the choices you make, they have genuine impact on gameplay. In the first level of Splinter Cell: Chaos Theory, as NSA ninja Sam Fisher, you'll overhear a pair of guerillas talking during a heavy thunderstorm. One of them describes how he witnessed his father's army unit get killed by American commandos during another thunderstorm decades ago, in Guatemala. The Americans spared the women and children.
GUERILLA: The Americans are not butchers. Their weapon is fear, and fear does not spread among the dead.
If you grab and interrogate the guerilla, he says:
GUERILLA: I knew you would come to finish what you started. Kill me. I long to see my family again.
This creates a context for the action you take next. If you kill him unnecessarily (butchering him), you're un-American. Plus, Sam Fisher's greatest weapon is fear - is it smart to throw that weapon away, or let the guerilla live to spread fear? Kill him and you bring a poetic symmetry to his story. Leave him unconscious and you change his life, maybe for the better.
Except none of that's true. Nothing happens to that guerilla after this level. Outside the game environment, you're just picking a shoulder button to pull. But if you grabbed the guerilla instead of shooting him because you wanted to hear more of his story, the words affected you. Whether you chose to knock him out or kill him, your choice was informed by his words. If they changed the way you played, the conundrum was real.
In an interview with 1Up.com, Chaos Theory writer Clint Hocking said, "I think those kinds of conundrums make Sam's character more engaging. You're kind of playing Sam the way you think Sam is." Thus, if we react to the guerilla's dialogue differently, you and I end up with different Sam Fishers. My Sam Fisher wouldn't kill a soldier he could spare. Yours might. We come away with different agents in the game. It makes your play experience that little bit different from mine.
The illusion is fertile enough for imaginations to take root in, to grow fan missions like the dozens at The Circle of Stone and Shadow, the Thief fan hub. Players are invested enough in the background to catalog and keep it in compendiums. Kieron Gillen's feature-length article about the terrifying Shalebridge Cradle level of Thief: Deadly Shadows shows how fear conjured into the game through journals and ghostly voiceovers - from words - lives on after the console is off.
For these players, the words have created a world. Like magic.