that's supposed to be the reaction if the body language was all good. it's like a subliminal message telling us "there's something wrong here" and yet, it's not blatantly said or acted.
my reaction so far after learning that kaoru is the real lord, "no wonder he said that" instead of "no wonder he was acting that way." and that's a bad thing in the part of the actor.
NOW all of these holds in the assumption that there was just bad acting on the part of matsuzaka (not bashing, he's just new to this thing) and the writers didn't care for the body language or whatever.
it's a completely different thing if the writers demand that no body language must be given so that no hints are released throughout the middle of the series. doing so would increase the level of surprise for the audience (and a little for credit for the writing)