When you are near the end of the second part of the movie, and Gandalf does his line "Sauron's wrath will be terrible, his retribution swift," look off to the far left where Eomer is supposed to be. That is not him. That is his stand in. I have watched the movie soooo many times, but I just caught that. You see King Theoden, Gandalf, Legolas with Gimli on the back, Gambling, and Aragorn, but the person to Theoden's left is definitely not Eomer! Unless he got a head transplant.