Michael Shanks is guest-starring in the 17th episode of the season, "99 Problems." He doesn't give too many spoilers, but he gives a good description of the plot and his character. Here's what Michael had to say:
Nrama: So what do you have coming up next?
Shanks: I literally just yesterday finished a guest spot on Supernatural. I had a relatively minor role in it, working with Jared (Padalecki) and Jensen (Ackles), and a few other actors, fighting some demons and throwing down with the boys. I think that will be out in a couple months. The episode is called, I think, '99 Problems'. I'm not featured quite prominently in it, but it was a lot of fun to play in that universe. It fit into my schedule quite handily. That's why I did it.
Nrama: Is there anything about the character you can tell us?
Shanks: Yeah. The character's name is Rob, and he's a member of the Sacrament Lutheran Church, and basically it's a township that has pretty much organized to fight the demon apocalypse that's coming. They're not running and screaming. They're hunkering down. They've figured out a way to fight and whatnot. The boys come across them and are quite enamored by they way they've organized themselves. They have a sort of militia...when a tragedy occurs in Rob's family he sort of takes a bit of a hardline approach when it comes to his beliefs. I'll leave it at that because it starts getting into spoiler territory beyond that.
From the casting call from a few weeks ago, I believe the family tragedy involves his daughter Abby. I'll give more details when I get them.