What I think I love best about all this, though, is the way that Dan is so caught up in the story they're acting out that he plays every scene in the episode literally as a scene from the play, down to the dialogue and blocking. As Ellen is introduced in the book -- to titters at the opera house, which infuriate Newland -- so too does he cast Rachel as this forbidden and transgressive delight. Their chaste conversation about what they can't do, in the clothing closet, turns into something physical -- just as in the play, in the carriage scene with Blair. Only it's not just Dan who can't tell the difference between Blair and Rachel: they've spent the last few episodes mirroring each other, bringing out the worst and more Blairish in each other, and it's destroying them both socially and personally. I think that Blair's change of heart -- and immediate downward spiral -- has more to do with the way Rachel has set herself up as this mirror, and the way Rachel has consciously been testing and crossing the lines of intimacy and privacy since she showed up: first with Serena, then obviously with Dan... And all along, Blair.
Blair's a child who pretends she's an old lady, Rachel an adult who acts like a high schooler. Blair who can game every system meets a nemesis that is the system, an entrenched authority, who not only won't stay on her side of the fence but actually comes across it and starts using B's weapons, like Gossip Girl herself... That iniquity in power is the thing that is killing Blair right now, I think, because it's dreadfully unfair and entirely inescapable. Rachel wins, because she's a grownup, end of story. And anybody who spends any amount of time under Eleanor Waldorf's thumb eventually gets a mullet or bulimia and violently rejects all authority... Rachel recapitulates Blair's relationship with her mother: arbitrary, unfair, incestuous and uncanny. Not to mention the fact that Rachel's little hissyfit last episode drove Harold out of the country. There's no revenge sweet enough for all that pain at once. I think that in the final analysis what we're seeing is that Rachel isn't on the show to have a relationship with Dan: she's there to draw Blair's true craziness out once and for all.
Which is where The Seagull comes in. It's one of those romances where everybody goes to a country house, like in all of Wharton and Austen, but it's Russian, which means somebody is going to be dead by the end of the play. (In fact, it's the origin of that Chekhov Law about how the gun better go off by the end of the play or else why have it onstage.) Kostya is in love with Nina and brings her this seagull he shot, which grosses her out because A) it's gross and B) she's in love with this other guy, who is in love with somebody else. So she basically doesn't even really see Kostya. Anyway, she goes off to be an actor and has a rough time of it, and everybody eventually comes back years later because it's a Russian play and they need to watch this old man die.