The only actors that I knew for certain before the movies came out would be Christopher Lee and John Rhys-Davies. Ian McKellen from X-Men and Hugo Weaving from Matrix are more iffy, as these film series overlapped LoTR. And I have seen very few movies since RoTK, that being the last one we saw in the cinema, and The Hobbit trilogy on DVD, so I'm pretty much ignorant of more current alternatives among actresses and actors for any of the roles.
Uma Thurman in her "Kill Bill" incarnation would have made an awesome Éowyn. But one would have to deescalate the persona from KB, otherwise Aragorn in the scene where Éowyn is doing sword practice and he interrupts her could very quickly find himself in a "HELP!!!" situation.
As an afterthought, PJ very much micromanaged the shooting, perhaps something that has become normal with the advances in cinema technology for cutting and pasting scene snippets (some of my favorite older films draw their appeal for me through their longish uncut shots).
So I am extremely hesitant to lay the blame for anything I consider weak in any scene at the doorstep of those involved in front of the camera. PJ chose the snippets, so what stinks is entirely his fault. And "weak" and "stinks" are of course very personal opinions, which others will not share at all, but then that works both ways. Should anyone take on the chore of collecting the sum total of gripes about scenes in any given movie, then probably every movie ever made would look like a waste of celluloid (a rather ancient term, as I am aware - it now would be disc space?).