These are some good suggestions. I'm sure any one of these actors could pull this role off just fine... But for some reason I think the only actor who could make this role super memorable and amazing is Cillian Murphy
(
http://www.imdb.com/name/nm0614165/)

Now, first off, I know he doesn't have a distinct resemblance to Ben Barnes (perhaps a little bit to Laura Brent), but that could actually work! Think about it. When Eustace sees Rillian in The Silver Chair, he doesn't recognize him as Caspian's son. So I think trying to find an actor that looks JUST like Ben Barnes isn't the best decision. Plus, I think Cillian Murphy's appearance totally conveys Rillian's Character. This leads me to my next point...
Secondly, I think Cillian Murphy can perfectly capture every aspect of Rillian. He has to be kind and hospitable, yet also be threatening. He has to be mysterious, capricious, and maybe even disturbing. Then, after the enchantment is taken off him, he has to be caring, noble, and trustworthy. It's a big change in character, and I don't know too many actors who can successfully pull it off. I think Cillian Murphy can. Wow, this sounds like an ad campaign, haha. But seriously, I am 100% for this guy.

Oh, one last thing, their names are almost the same! Cillian and Rillian. Wow! It's destiny. Haha.
