What exactly happened to Thranduil's face while he was arguing with Thorin?
In the heated discussion between Thranduil and Thorin, for a moment, Thranduil's face suddenly turned very deeply scarred & wounded and then immediately returned to normal. What was this about? Is it a reference to anything that was cut out of the movies? (I have not read the books, sadly, but am very curious about this.)
Look at that. You only have to wait a million minutes and someone'll come along with a canon answer...
I found this source but i don't know whether it is what you are after.
Thranduil’s sudden scars reflect a little emphasized of Tolkien’s lore: elves’ “Fëar” (a metaphysical concept analogous translatable as “soul”) occasionally influences the “Hröa” (the fleshly, physical body), particularly under moments of extreme stress. This can manifest as extreme physical changes that reflect the mind’s state, in this case deep war scars.
Edit : I have also found this reference which seems to state that Tolkien made no reference to the Scars on Thranduil's face in the books.
Thranduil's scars are decidedly non-canon. According to Tolkien's texts, the last battle Thranduil saw (up to setting in The Hobbit) was that of the last alliance at the end of The Second Age. However, in the 1900's of the Third Age, according to text, the "serpents of the north" were fought and mostly slain. In all likelihood, Peter Jackson is taking creative liberty with the story. On a more symbolic note, the scars could represent the pain Thranduil has endured in war (he watched his father die in the battle of the last alliance).
That is a good discussion. It is what I was looking for.. Rather, I came across this while Googling the same, but there are a lot of speculations in it. I was looking if someone over here could give a more specific answer based on the books or so...
so was it his fear or anger showing up on his face, or was it a past scar that was simply resurfacing because of the topic of a Dragon?
Pretty sure this was during the scene where they were talking about Dragons and how dangerous they were... perhaps something about Dragon fire... And he gets angry saying Don't you dare tell me about dragons / dragon fire. I believe that Thranduil ACTUALLY has a scarred face due to battling dragons and that he uses some sort of illusion magic to hide his scarred face. If they took the lore from Tolkien regarding Fear and Flesh that's a plausible theory as well.
I'll have to check HoME later on to see if the first statement can be corroborated: even if so, Jackson wouldn't be legally allowed use it, so I highly doubt if it's in reference to anything like that. The latter, more prosaic explanation seems most likely: maybe there'll be something in the EE or the 3rd movie?
@DoctorWho22's theory is what I took away from the scene. Whether or not it's in the books (I need to re-read them, it's been *years*), I believe Thranduil was scarred in battle with a dragon, and that he now uses "elf magic" to keep his scars hidden. In arguing with Thorin, he either loses his focus in his rage or, to emphasize his point, he deliberately "turns off" the illusion; either way, what we briefly see in that scene is the scars his body actually bears from "dragon fire", as he is speaking of at this moment. Won't post this as an answer, though, because I have nothing to back it up.
I cannot begin to scribe how often in fantasy novels do elves use illusion magic to alter their appearance, particularly those of royal blood. @DoctorWho22's theory was what I assumed as well, mostly because I've encountered the situation so many times in so many books.