10 Small Details You Only Notice Rewatching Supernatural
2. The Introduction Of Castiel
The character of Castiel was introduced in the first episode of the 4th season. Castiel is the angel who saved Dean from Hell... or as he puts it, " I'm the one who gripped you tight and raised you from perdition."
While, it wasn't until the 4th season that viewers learned that angels officially existed in the Supernatural universe - the first time we hear Castiel mentioned was actually in 'Red Sky at Morning', the 6th episode of the 3rd season.
In it, Sam reads from a book in order to try and summon a ghost. The first part he reads is, "Asyel, Castiel, Lamisniel." Invoking the name of the angel Castiel this early on sure feels like some subtle foreshadowing for detail oriented fans re-watching the third season.
It seems that it was entirely unintentional. Series creator, Eric Kripke, tweeted that when they needed a new character name for the 4th season, he googled "Angel of Thursday" since the show aired on Thursdays. This impromptu search gave him the name Castiel.
Kripke appears to be unaware that the name had been used in the season prior. There's a reason for this. The section Sam reads is actually a quote from the real book Forbidden Rites: A Necromancer's Manuel of the Fifteenth Century. The spell used for "constraining a man, woman, spirit, or beast" in the book just happened to use the angel's name as part of the ritual.