You folks are both in error. Sienna Miller was not in LOTR. Cate Blanchett was, of course, as Galadriel, the Elf Queen, holder of one of the Rings of Power. Liv Tyler, who was mentioned by MadIvan, was the female lead of the series, Arwen, daughter of Elron half-Elven, the great love of the King. Ms. Tyler is the daughter of a rock star, and comes out of that milieu.
You folks are both in error. Sienna Miller was not in LOTR. Cate Blanchett was, of course, as Galadriel, the Elf Queen, holder of one of the Rings of Power.
Didn't I just *say* that? :-)
Cheers!