A year late, but I just showed my friend the episode (he's just caught up on all the series).
I think what's really underrated in all the reaction stories is how successfully they made you drop their guard before shit went down. I wish I'd had recorded his reaction, cause he fully bought how happy and gleeful the proceedings were up to that point, pointing out how things turned around for silly Edmund Tully.
And then the shocked reaction afterwards. Talk about blindsided.
And back when the episode aired, my friend at the time? She moved some furniture around when that happened, cause Robb and Cat were her favorites. She thought they were setting up a huge fight for next season.
It's funny how you can see a commonality in body language in all the reaction videos where people perk up in alarm, then jerk in shock when it happens, like the little exclamation mark in MGS games.