Jackson changed his mind about the
Arwen character sometime either during or just after the release of the first film. He had intended her to be at
Helm's Deep but edited her out (although it is possible to get a "glimpse" of her there). He wanted to make her a strong character in the beginning, hence the Arwen Warrior Princess bit in FOTR. However, I think he got a considerable amount of negative reaction from that particular depiction and so he decided to do the "love interest" bit, making her more vulnerable and developing the non-existent "conflict" with her father
Elrond over her choice of
Aragorn as her lover and her decision not to go into the West. Then the whole middle of TTT has that strange
elvish "dream sequence" that is never fully or properly explained. It appears as if Arwen is leaving for
the Grey Havens, but, of course, we know that she does no such thing!
I don't know if Jackson ever truly decided where he was going with a plot thread that he had decided to greatly
overdevelop with the excuse that the audience demanded a love story! In the end, all that actually
was "developed" is rampant confusion even, apparently, among the FADs and Filmists.