There are two stories running in ‘The Big Empty’, both of them dealing with emptiness, either literal or metaphorical. There’s Dean and Sam, struggling with the emptiness left behind by too many losses and taking it out on each other (and Jack – who is also struggling with the emptiness of being orphaned and not knowing who you are).
And there’s the literal Big Empty, where Castiel awakened at the end of last episode to find himself alone.
When this episode starts, however, Cas finds himself alone no longer. There’s an entity there with him that has his face and form, but it’s not exactly good company. In fact, it’s pissed as hell that when Cas woke up, he did too. The Empty is apparently the place where angels and demons go when they die, and they pretty much just slumber there, which means the Entity can slumber too. He loves sleep almost as much as I do, and that’s really saying something. That Castiel is now keeping him from his cherished sleep is making the Entity very cranky – which he takes out on Cas with a lot of physical abuse. Ouch.
Supernatural has played with characters confronting different versions of themselves before – Dean confronting future!Dean in ‘The End’ memorably comes to mind – so Cas confronting Entity!Cas isn’t unprecedented. I was caught off guard, though, by the way Misha Collins plays the other version. Because he’s very very different! I wasn’t sure whether to laugh at Entity!Cas or be chilled by him, or maybe a little bit of both. He seemed to have a little bit of Alastair and a little bit of Zachariah and possibly a little bit of Balthazar in him too. He almost seems friendly, or at least amusing, until he punches you in the stomach. Again, ouch.