This moment was so great, because you can tell that Beckett isn’t only shocked by what the fire investigator is saying about the man being her partner. It’s like in this very moment it hits her that her and Castle could be in this exact position at any time, and it’s terrifying. I know it’s not addressed that often, but I love when the danger of what they both do is thrown into the scenes, even in the tiniest of ways. It always makes for great moments between the two of them, like this one. The look on Beckett’s face is one of those that just hits you. At any moment one of them could get hurt, or worse. There’s no guarantee that they’re both going home at the end of every day, and I think this look on Beckett’s face, this little moment, was the icing on top of the cake of what was a super amazing episode for Castle and Beckett.