Great stuff from that redditor. I have to say that I was completely baffled by the Arya-Sansa game of faces scene. Sansa had no idea what was going on, kept asking basic questions about the face which Arya could have easily answered, but instead chose to give Sansa a dagger. (WTF?)
Viewed from the prism of Arya playing a double game on Littlefinger the whole time, it totally makes sense that she wouldn’t explain that she was a faceless assassin, or Littlefinger would have been riding to the Vale that same night.
Usually, TV shows ask us to believe that the only awareness of the characters is that which is shared with the audience (in the same way we don’t believe someone is dead unless we see it on screen). In this case, all of the sister to sister plotting and confirmation with Bran happenned off screen.
So the writers did the exact opposite of what we see in most shows, where the hero is walking somewhere and be completely surprised by a bad guy, even though a normal human would see something in their peripheral vision and start to react before the attacker made contact with them.