Looking at some of that material, fans can gather evidence and piece together a few potential scenes.
The scene I want to piece together today centers around Thorin Oakenshield. For as much as the King Under the Mountain gets rebuked in the official trailer, he does get one thing to say for himself: "Everything I did, I did for them." Since he's most likely referring to his nephews Fili and Kili, I think I know exactly what the context of that line is.
First of all, Thorin isn't wearing any of his kingly attire in the shot where he says it. This means that the scene either takes place before Fili, Kili, Bofur, and Oin arrive at Erebor or after Thorin overcomes his "Dragon Sickness" and decides to join the battle for the climax. He doesn't seem to be wearing any armor, so the first theory is probably the case.
Second of all, the trailer shows a shot right before that where Thorin appears to be standing among Erebor's treasure hoard without his crown on. If memory serves me correctly, I don't think that's a scenario we ever saw in the theatrical cut. Since he doubtfully would have gone back to the treasure after casting away his crown, that shot most likely takes place before the four dwarves in Laketown arrive at the mountain. Therefore, the shot where Thorin says his line probably occurs around that same time.
I found it a bit jarring in the theatrical cut when Fili, Kili, Bofur, and Oin arrive at Erebor and Bilbo just runs up to them saying, "FYI, Thorin's a totally different person now." It's understandable that this is how those characters would be informed of Thorin's transformation, but it's also how the audience is informed of it. No more gradually watching his character spiral downward -- he's just completely corrupt now. Since Bilbo mentions that Thorin has been counting the treasure in the mountain for days, we can assume that a scene of that downfall was originally included in the film and might return in the Extended Edition.
So how will it occur?
Well, let's look at Thorin's introduction in The Battle of the Five Armies. As Bilbo and the dwarves watch Smaug attack Laketown in the prologue, we see Nori trying to console his brother Ori. After that, we see Gloin looking dreadful and Bombur looking horrified. I always interpreted that as Gloin and Bombur realizing that their brothers, Oin and Bofur, are still in Laketown and caught in the middle of the carnage. Given that, I see Thorin's reaction, standing away from the group with his back to Laketown, as him realizing that Fili and Kili are also caught in the attack and not being able to watch.
It's likely that the Company's next scene in the Extended Edition will be a new one where they discuss their concern for the dwarves left in Laketown. Since Thorin isn't the most optimistic dwarf, he'll probably be convinced that Fili and Kili didn't survive Smaug's attack. That will prompt him to dismally say his line, "Everything I did, I did for them."
You can imagine what a horrible state of mind that would leave him in. He thinks his nephews are dead. The whole reason he set out to reclaim Erebor was to build a better future for them, and now the whole quest has been for nothing and he no longer has a purpose. And it's pretty much all his fault that Fili and Kili were left behind in Laketown to begin with. Adding the fact that all of his loyal followers are now mourning over two other dwarves that he left behind, it's a wonder Thorin didn't jump right off that cliff he was standing on at the start of the film.
It's pretty easy to guess where things might go from there. Thorin will probably wander away from the others in his depression, come across the treasure hoard, and literally lose himself in it. As we see in the theatrical cut, he gets so lost in it that when Fili, Kili, Bofur, and Oin turn up alive in Erebor, he barely even cares. The only thing he wants to do is show off all of his new treasure to them.
Let me just state for the record how much I hate the concept of "Dragon Sickness." It seems to be presented, at least in the movies, as a mental disorder caused by something in Erebor's gold that descendants of the dwarf king Durin are especially susceptible to. Being corrupted by greed has nothing to do with who those descendants are as people; they have a certain genotype, so they're going to catch that disorder from the contaminated gold. It's the same problem people have with the Midi-chlorians from Star Wars -- it reduces what should be a character trait to a condition.
Having Thorin succumb to "Dragon Sickness" because of his grief over his nephews, however, would fix that. He wouldn't have fallen to temptation because of his bloodline. He would have fallen to it because he lost hope and momentarily lost the will to resist it. That would have everything to do with who he is as a person. It would even make us wonder if he might have endured had Fili and Kili been with him the whole time.