When it comes to our protagonist, it's a little hard to separate thought from action; yes, we hear him narrate events, but each step, each futile gesture in the dark, is accompanied by thoughts, right? Still, the narrator is defined by his unwillingness to give in. Even in his terror he manages to escape from the pendulum; even when the burning walls are pushing in on him, he considers dashing himself against them – he just will not resign himself to the pit. Having said all this, a much simpler example presents itself. General Lasalle is defined by a single action, a single movement: the thrusting forward of his outstretched arm. Done and done.
The narrator is defined by his surroundings in the most basic way possible. He is a prisoner because he's been thrown in prison. As he himself says, though, the curiousness of his prison is equally important. We never know what crime the narrator has committed; he neglects to mention even the smallest detail. And yet the dungeon itself, with its curious design and devious mechanisms of torture, sets him apart from those heretics burned at the stake. He's a special guy simply by virtue of his special cell.
This is all you need to know about the inquisitors: "I saw the lips of the black-robed judges. They appeared to me white – whiter than the sheet upon which I trace these words – and thin even to grotesqueness; thin with the intensity of their expression of firmness – of immovable resolution – of stern contempt of human torture" (1). You know that golden oldie, "It's In His Kiss"? Well, with the judges, it's in their lips.