[Elladan and Elrohir]

Speaking of Andy Serkis, who else thinks he's underrated in this film? Maybe not underrated persay, but he doesn't get as many kudos as he deserves. Everyone, EVERYONE raved about him after TTT, but when ROTK rolled around, it was like, "Oh, yeah, Gollum's awesome again. No big deal."
Serkis may not get quite as many opportunities to shine in this movie as in the last one, but his performance never lets up for a second. And the CG on Gollum's face is actually a little bit improved even over TTT.

Of all the characters in all the films, Legolas is probably the easiest to act. I think Orli does an awesome job in the role, and really becomes Legolas, but I think there's probably also plenty of others who could have done the same. I don't know that Orli is really that great of an actor, though the only other thing I've seen him in is Pirates of the Caribbean. But I heard he was rather unimpressive in Troy and Kingdom of Heaven.
I joke with my brother (Elrohir) that Legolas only has one facial expression throughout the entire three films. OK, it changes once per film.