I love the innocent involvement of David. The first time he met Barnabas, he thought Barnabas was a ghost, and now when he sees Maggie dressed up like Josette, he thinks he's looking at the ghost of Josette - and of course that's even more pleasing than his mistake about Barnabas, because we've been seeing and hearing about Josette for nearly a year, whereas we first saw Barnabas’s portrait only a few weeks ago.
And then I started thinking about the changes in Collins family history that the writers had decreed in order to make David's favorite ghost into Barnabas’s lady-love, and suddenly it didn't seem as magical as it had before. Never mind. I still love it.