Here you may discuss interesting facts about The Lord of the Rings movies including, including the difference between the books and the film adaptation.
Well, yes, i understand that Arwen was not meant to do that much in the movie, but it still worked out well anyway.
Arwen was only shown in the book as a hopeful lover, and I'm glad that they portrayed a bit more of her in the movie. She looks so strong and graceful especially when she unsheathes hadhafang in defence of Frodo.
Personally, I actually go for Arwen in the movie because she just looks even more noble and love-deserving through that. I agree that it's totally unlike the books, but Peter JAckson made good decisions as we can still understand the story of the Ring as Tolkien put it.