Taking to Facebook, the director revealed that Lilly -- who played sassy fugitive Kate Austen (aka "Peaches") on the 2004-10 ABC show -- will play an elf that does not appear in the J.R.R. Tolkien novel on which the movie is based.
Jackson also announced that Barry Humphries, the Australian actor best known for playing the campy Dame Edna Everage character, will play the Goblin King. The role will be CGI'd "in the much the way Andy Serkis created Gollum," wrote Jackson.
"Evangeline Lilly will be playing a new character -- the Woodland Elf, Tauriel," wrote Jackson. "Her name means 'daughter of Mirkwood' and, beyond that, we must leave you guessing! (No, there is no romantic connection to Legolas.) What is not a secret is how talented and compelling an actress Evangeline is; we are thrilled and excited she will be the one to bring our first true Sylvan Elf to life."
Lilly has a tenuous connection to Middle Earth: She dated "Lost" costar Dominic Monaghan, who appeared in Jackson's "Lord of the Rings" trilogy as the hobbit Merry.
Finally, Jackson said that English actor Benedict Cumberbatch will voice Smaug the dragon.