I'm not sure where they are going to fit Legolas into the movie.
The appendices, from which a lot of the extraneous material will be taken, states that Legolas was the son of the Wood-elf king, Thranduil.
While not named in the HOBBIT, Thranduil does appear: he's the elf-king in Mirkwood that puts the dwarves in his dungeon while Bilbo skulks around invisible with the ring (and eventually dumps them all into barrels to be floated to Dale). The latter scene is in the early trailers.
So Legalos had to be around Thranduil's palace somewhere during that time, as his son; I imagine he'll bump into Bilbo (literally, haha) and keep his secret or something like that. Also, the Battle of the 5 Armies will have the elves, too, presumably the wood-elves among them, so Legolas gets to fight later.
Radagast the Brown is in some of the early trailers: when Gandalf leaves the dwarves at the entrance to Mirkwood, it is to gather the White Council (Saruman and the two other secret ring-bearers, Elrond and Galadriel; Gandalf has the third elf ring) to drive out the Necromancer (who is an early form of Sauron). Gandalf met Thorin's father in the dungeons of the Necromancer while checking to see if the Necromancer was Sauron returned, which is where he gets the map at the beginning of the HOBBIT with the secret entrance. Thorin's father had a dwarf ring, but the Necromancer took it away from him before putting him in the dungeon, but apparently didn't find the map. Orcs make sloppy jailors, apparently, lol.
I suspect Jackson will bring up Thorin's father's story through Gandalf at the beginning of the HOBBIT, just like the book. That sets up the side-plot of driving the Necromancer out of Mirkwood (and into Mordor, eventually), tying all the films together, and allowing Saruman, Elrond, and Galadriel to be in the story as well.
My best guess is that Jackson will follow the pattern of the LOTR films, and have a skirmish end the first film (the Warg attack after they escape from the Goblin king?), the White Council battling the Necromancer at the end of the second film, and of course the Battle of the 5 Armies at the end of the final film.
Should be a cool magical battle between the White Council and Sauron in his Necromancer form! Radagast is a very minor figure, even in the appendices, but he talked to animals, so I suppose that's him with the furry critters in the trailers, sending a message to Gandalf that the Necromancer might be Sauron returned. There were two other wizards in the appendices, part of the five Istari with Gandalf, Saruman, and Radagast, but they weren't even named, and presumably were away down South or East on other errands.
(Can you tell I've read and reread the books since childhood?

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