As we all know, Peter Jackson has greatly expanded The Hobbit. What started as a story from a short novel has turned into three epic movies and he has included a variety of elements from other Tolkien works, like stories from the Silmarillion, into the movie. This is where the side-story of the Necromancer that Gandalf is investigating comes from.
Some may assume this is also how Legolas and the new character of Tauriel (Evangeline Lilly) have come to be wedged into the Dwarves' tale, but that isn't quite so, as Peter Jackson explains:
People always ask about Evangeline’s character Tauriel and why we felt the need to create her. But in The Hobbit novel, [the dwarves] are captured by the elves and they escape in the barrels. And it’s a memorable part of the book but the Elf King is not even named. He doesn’t have a name. And it was only later on that Tolkien decided it should be Thranduil and he also decided he should have a son when Lord of the Rings was written 18-19 years later. He created the character of the son of the king.
So you’ve got material there but you can’t have a scene in a film that’s a memorable scene with just one person as the Elf. We wanted three Elven characters. That’s the thing too, to create characters and have conflict with each other and they have different agendas. Thranduil, Legolas and Tauriel are all on different flight paths which makes it much more interesting ability for Fran [Walsh] and I to tell the narrative through their eyes.
As much of a reader as I am, I've only read The Hobbit and Lord of the Rings one time long ago and would never have known that the elven king wasn't named, nor did I connect that same king later having a son would have been Legolas anyway. The inclusion makes a lot of sense, but Legolas being described as the "link" by Evangeline Lilly less so.
Peter, Fran [Walsh] and Philippa [Boyens] brilliantly tied the last Hobbit film to the first Lord of the Rings film through the one character would could do that, which is Legolas.
It will be interesting to see why Legolas is named as the link when there are already a few existing obvious people connecting the two trilogies. Perhaps something to do with the Ring or the return of Sauron will touch on Legolas' side-story by the time the third movie has wrapped. What do you think?