"CARTOONS AREN’T ALWAYS FUNNY — Castiel (Misha Collins) tells Sam (Jared Padalecki) and Dean (Jensen Ackles) that he’s decided to become a hunter like them. Sam and Dean aren’t sold on the idea but agree to investigate a case Cas found where a man’s heart literally burst through his chest. The guys discover there has been more than one odd murder in the small town and all of them resemble cartoon deaths."
This episode features a guest appearance by Mike Farrell from M.A.S.H. who plays an old Winchester family friend who the trio also try to save. During a visit to the Supernatural set in September, we watched Mike, Jared Padalecki and Misha Collins film a scene for this episode that involves Castiel and Sam jumping into the head of Mike's character in order to try help him in a cartoon-type world. In the scene we saw Sam and Castiel themselves aren't animated, but everything around them is. This is very different from previous Supernatural episodes, but the concept and execution look great! We'll have some interviews from the actors for you as the episode approaches.
I left this out because it's mostly old info except for the bolded part.
*Paul Edwards directed the episode written by Andrew Dabb (Source)