So, I have this theory about Arthur that I’m rather fond of. I think he’s known about Merlin’s magic since the end of season one (he remembered Gwen talking to him when he was dying, so why not Merlin killing the beast/trying to heal him?) and that’s why their relationship deteriorated a bit. Arthur was in denial for at least the entirety of season two, and now, perhaps he’s waiting for Merlin to trust him enough to tell him his secret.
So. I rewatched this clip a few times, and I started to think that something about the way he looks at Merlin when he says Arthur would be surprised, and his tone of voice, and the way he looks away afterwards pointed to him already knowing about Merlin’s magic, but perhaps with a touch of denial, and/or hope that perhaps Merlin would finally reveal his magic. But, he was disappointed, yet again.
Rewatching this video I just made again made me wonder how the hell I got that out this. :/