ThroughoutThe Hobbitbook and its movie adaptations, Thorin’s character is one of the most difficult to pin down. He is oftentimes brave and courageous,willing to sacrifice himself to protect the things he holds dear, and other times obstinate and unyielding, and too proud to accept help or admit his own mistakes. He is by no means a bad guy, in fact, he is one of the best heroes in the story, but he is a dwarf nonetheless, and a dwarf who has been wronged by many great foes in the past, so he has developed aself-righteousness to his claimas king under the mountain.
The quintessential test comes for his character in the scene in which the other dwarves are readying themselves for a battle that they know they cannot win, upon Thorin’s orders. Meanwhile, Thorin pulls Bilbo aside and tells him that he thinks one of them, his own kin, his own blood, who have traveled with him, protected him, battled alongside him, and given everything to him, is a traitor, and has taken the most coveted jewel of their kingdom: the Arkenstone. Bilbo’s horror and despair at this moment is palpable, and it completelyredefines their relationshipand Thorin’s character as a whole.

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There are some incredibly complex things happening in this scene that culminate in making it one of the most spine-chilling moments in the film trilogy. The first thing is the background music, which takes on an eerie and suspenseful atmosphere. The fact that Thorin is already talking about suspicion and betrayal in the moments before, combined with the fact that the audience knows that it is actually Bilbo who has the Arkenstone about which Thorin’s accusations of betrayal rest, ramp the tension up to the point when Thorin says “I will not part with a single coin. Not one piece of it.” This is the climactic line, in which Bilbo, and the audience through his eyes can truly see how far Thorin has succumbed to the dragon sickness, and howhis own greed and lust for powerhas driven him mad, just as Smaug warned it would.
This is a very clever directorial decision on behalf of Peter Jackson and the talented writers and creators at Weta studios, because they have taken this line about not parting with any of the gold in the mountain directly from Smaug and given it to Thorin. As the two both say the line, their characters and their associated evils become linked. The sound team also had a hand in creating this illusion in the audience’s mind, because they layered some of the deep notes, and snake-like hisses and slithers that they used to create Smaug’s voice over Thorin’s, so that fans can actually detect the serpentine sounds of dragon-sickness when Thorin says it.

This is also mirrored by Richard Armitage in the body movements he uses to portray the scene, the way he moves his head from side to side as if he is writhing and coiling like a dragon would. He learned this technique from watching Benedict Cumberbatch’s performance of Smaug when he was recording the interaction with Bilbo earlier in the trilogy, and paid particular attention to the delivery of this iconic line so that he could recreate it later, further linking Thorin withthe terrible fate of Smaug.
It is at this point that Bilbo realizes that Thorin isbeyond his own redemption, and that it is going to take something drastic to prize the dwarf away from the gold in Erebor. Of course, the only thing that holds enough sway over him is the Arkenstone itself, which Bilbo claims as his fourteenth share of the treasure and uses as a bargaining chip forthe people of Lake Townand the elves of Mirkwood to actually get the compensation they deserve after Smaug destroys their home and kills half of their people.
This line is essentially the turning point of the story, and the point at which there is no return for Thorin. His madness drives him to almost throw Bilbo off the parapet, and he doesn’t manage to reconcile with the hobbit until right at the very end, when he is dying on the battlefield, and he asks Bilbo’s forgiveness for the cruel words that were exchanged between them. Thorin returns to himself, free of the dragon-sickness at last, and dies nobly, but that last moment of trauma, both watching his friend surrender to greed and madness, and then watching him die before he ever truly has a chance to rectify it, stays with Bilbo for the rest of his many long years.