When Bilbo fights against a giant spider in Mirkwood with his sword, he hears the spider say "It stings, it stings". This gives the name to Bilbo's sword, which years later he would give to Frodo in The Lord of the Rings: The Fellowship of the Ring (2001).
During filming, Ian McKellen had to spend hours in a box with nothing but a microphone and pictures of the dwarves for company (the footage would be edited in to make him look taller than the dwarves), and was so upset by this that he exclaimed "This is not why I became an actor!" The microphone was still on and everyone on set heard him as a result.
The only time Legolas blinks is when he realizes he has been wounded. This is in keeping with the character, as the only time Legolas blinks in the Lord of the Rings trilogy is when he is strongly surprised.
Martin Freeman and the Elvish actors all play characters who cannot grow facial hair. The actors, however, not only grew facial hair, but had a complete five o'clock shadow by the end of a day's filming, even if they were clean-shaved at the beginning of the day. This shadow appears blue on film. Rather than shave repeatedly during the day, the actors had orange make-up applied, which canceled out the blue shadow.
Bilbo steals the keys to the prison from an Elf addressed as Elros. Elros is also the name of Elrond's brother. The two brothers had both human and Elvish ancestry on both their mother's and father's side, and as such were perfectly half-Elf and half-Man. While Elrond chose to be an immortal Elf, Elros chose to become a mortal man. He thus became the progenitor of the Numenorian race, and an ancestor of Aragorn.
It took one processor-week to render a scale from Smaug's body. In other words, if a single computer processor were used, each scale would have taken a week to render. Fortunately, Weta Digital used a large server farm of multi-processor machines for its effects rendering.
Aside from providing the voice of Smaug, Benedict Cumberbatch also supplied motion-capture performance to portray the dragon. However, since Smaug's body is impossible to motion-capture fully, only Cumberbatch's face, upper body and arms were captured and the rest of the body was completed in keyframe animation.
Benedict Cumberbatch studied iguanas and Komodo dragons at the London Zoo's Reptilian House to prepare for the voice of Smaug, aiming for a tone that would "bridge between animal and human, a deep and rasping guttural dryness to the voice."
After the barrel riding sequence, as the Orcs continue to search for the Dwarves, one of them dips his finger into a pool of blood on the ground and tastes it, spitting in disgust and says "Dwarf blood!". This mimics a scene from The Lord of the Rings: The Two Towers (2002) where Gimli dips his finger in a similar fashion into some blood left on some leaves and tastes it, only to spit out in disgust and say "Orc blood!"
Thranduil's sudden scars reflect a little-emphasized facet of J.R.R. Tolkien's lore: elves' "Fëa" (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.
The sudden scars on Thranduil's face are a creation of the film adaptation. According to J.R.R. Tolkien's texts and books, the last battle where Thranduil fought was the last alliance at the end of the Second Age (about 3000 years earlier), where Sauron was defeated. The scars, a creative liberty of the writers, represent a symbol of Thranduil's endurance by war.
This is the only movie in the Middle Earth series directed by Peter Jackson to not feature Frodo and Gollum and the first of the many Middle Earth films including animated ones not to feature Elrond leaving Gandalf being the only one to appear in every single film based on Middle Earth series written by J.R.R. Tolkien as Bilbo doesn't appear in The Lord of the Rings: The Two Towers (2002)
The 48 FPS 3D movie was shipped to some theaters via a standard SATA hard drive. It was shipped with a security code that prevented the content from being viewed or copied until the code was released by the distributor about 24 hours in advance of the initial release time of the movie. The 48 fps 3D version of the movie occupied 639 GB of data on the SATA hard drive. The theater in question could have downloaded the entire movie via a secure satellite link, but the download would have taken more than a full 24 hours. Given that 96 frames per second are required for the 3D picture (48 fps for each eye), the data size for each frame for each eye is about .66 megabytes.
Robert Kazinsky was cast as Fili and had filmed a few scenes, but left the project and returned to England about a month after filming started due to personal reasons. He was replaced by Dean O'Gorman.
While the novel and the previous film The Hobbit: An Unexpected Journey (2012) are mostly told through Bilbo's point of view, this film shifts the perspective to mainly Gandalf and Thorin with little involvement from Bilbo in many major scenes.
In the "Barrels out of Bond" sequence, there are two instances where Thorin throws a sword at an orc to kill them, first to protect Balin when an orc jumps at him from above, and second to stop an orc from attacking Legolas from behind. If you pause the film at these times you can see that in both instances the sword he throws has the same design as Balin's sword, which he used in both films until was taken by the Wood Elves upon their capture in the forest.
The trivia items below may give away important plot points.
According to Kili, the black stone he carries was a gift from his mother to remind him of his promise to return to her. When he is looking at it in Thranduil's dungeons, runes can be seen on it, which translate as "inikhde", a word in Khuzdul (the dwarvish language, created by J.R.R. Tolkien) which means "return to me".
Gloin takes great insult that the portrait of his bearded wife is mistaken for being his brother. In the extended edition of The Lord of the Rings: The Two Towers (2002), his son Gimli notes that Dwarf women are often mistaken for men, on account of the beards.
When Bilbo first confronts the dragon Smaug, he explains who he is in various nicknames. He says that he comes from "under hill" to which Smaug replies, "Underhill?". This is the name by which Frodo Baggins covered his true identity "The Lord of the Rings" as per Gandalf's request.
Thranduil makes it clear that he doesn't consider Tauriel to be worthy of his son, Legolas, but the film avoids explaining why. In the books, it is established that the wood-elves of Mirkwood are Silvan, but Thranduil and his family are Sindar ('Grey Elves'), a more noble ruling class of elves. This difference is also alluded to in the film by the fact that Thranduil and Legolas are blonde, while all the other Mirkwood elves have darker hair ( although Legolas' hair colour was never declared in the books).
When Kili is being saved by Tauriel from the Morgul-tipped arrow, Kili sees Tauriel surrounded by light. In The Lord of the Rings: The Fellowship of the Ring (2001), the first time that Frodo, similarly hurt by a Morgul knife, sees Arwen, she also appears surrounded by light.
There is a fan theory that the Arkenstone is actually one of the lost Silmarils, from J.R.R. Tolkiens "The Silmarilion." Changes in the visual design of the Arkenstone from how it is described in the book seem to indicate Peter Jackson (or one of his design team) subscribe to that theory. Since they do not have film rights to that book, explicit references to its story and characters do not feature in the movies.
When Legolas fights against Bolg and the rest of Orcs in Laketown, he uses Thorin's sword ("Orcrist", the Goblin-cleaver), which he kept after disarming Thorin and the Dwarf company when they were captured in Mirkwood.
The first meeting between Gandalf and Radagast takes place in the mountains of High Fells of Rhudaur, to investigate an underground cave. They find some tombs desecrated, and its iron doors blown out. These are the tombs of the Nazgul (the ancient nine kings of men), mentioned by Galadriel (Cate Blanchett) in The Hobbit: An Unexpected Journey (2012). Rhudaur is a Sindarin word (a fictional language created by Tolkien) and means "Eastern Forest".