Joseph Campbell's Hero's Journey
Joseph Campbell was a comparitive mythologist who has best become known for his 1949 book The Hero with a Thousand Faces. In this book Campbell presents the concept of an archetypal hero whose journey (or 'character-arc') is repeated throughout worldwide mythologies and religions. This journey is termed the "Hero's Journey"
Eric Kripke has confessed his fascination with the Hero's Journey and his exploration of it within Supernatural (Jensen Ackles reveals this in a behind-the-scenes Supernatural special). George Lucas is another director who has publically acknowledged his use of the Hero's Journey arc (in Star Wars).
Below is a reading of Season 1 (and speculation of beyond) within the Hero's Journey arc, as speculated by Leandra. Titles and subtitles refer to Campbell's dividing up of the Journey into chapters in the first part of The Hero with a Thousand Faces.
The Adventure of the Hero
Chapter I: Departure
1. The Call to Adventure
"Dad's on a hunting trip, and he hasn't been home in a few days."
Sam is the hero and Dean is the one calling him into the adventure.
2. Refusal of the Call
"I swore I was done hunting." - Sam's reluctance to go with Dean, his return home for the law interview instead of continuing to search for John with Dean.
3. Supernatural Aid
Once the hero has comitted to the quest ("We got work to do!"), his guide appears or becomes known - in this case, Dean. He's the one protecting Sam, he's the one who "always has his back".
4. The Crossing of the First Threshold
Leaving the hero's known world behind - Sam leaving his 'normal' college life and heading back on the road; in Wendigo, immediately after the Pilot, he literally heads into the wilderness.
5. The Belly of the Whale
Episodes: 2-6, coalescing in 1.11 Scarecrow (episode)
The hero finally accepts what he has to do, he accepts his new self - Sam goes to find Dad on his own, then comes back to Dean of his own accord; accepting his lot on this journey as a hunter.
Chapter II: Initiation
1. The Road of Trials
episode: all episodes
The hero undergoes a series of tests and tasks - Sam (and Dean) coming up against a number of foes and situations they must overcome each episode.
2. The Meeting with the Goddess
Hero experiences a love that is unconditional. It is suggested that this unconditional love is represented by a soul mate, the hero's other half - this person can also be someone who is an opposite of the hero. This role is filled by Dean - there's no love more unconditional than the love the Winchester brothers have for each other, and we are reminded of this a couple of times throughout Season 1.
3. Woman as the Temptress
Within Season 1, this could be characters such as Sarah (1.19 Provenance (episode)) or Lori (1.07 Hookman (episode)) 'luring' Sam away from the quest and back to a 'normal' life. In the grander arc, it could be something like being reunited with Jessica. The temptation doesn't need to be a woman at all, despite the name of this stage, but could come in any type or form.
4. Atonement with the Father
episodes: 1.16 Shadow, 1.20 Dean Man's Blood, 1.21 Salvation, 1.22 Devil's Trap
The person must confront the ultimate power in their life. Ostensibly John could fit into this role (for Sam), but also it could be "the only thing that's ever been important to us" - their quest for the Demon.
episodes: 1.17 Hell House, 1.18 Provenance
This is usually the part in a movie (right in the middle of one), before the really bad bad exciting things start to happen. It's the stage where the main characters usually fuck their brains out. Campell describes it as "dwell(ing) in the groves of the wish fulfilling trees, drink(ing) the brew of immortality, and listen(ing) everyhwere to the unheard music of eternal concord." In any case, it's the part in a story arch, where the character is rewarded, where they can take a break from their quest, or where their quest allows them to take a breather.
6. The Ultimate Boon
Achievement of the goal. Season 1 builts up to this ultimate goal - killing Celine Demon in the Season Finale, and it doesn't work out. Season 2 will probably revisit several stages of the hero's journey.
Chapter III: Return
The following steps in the Hero's Journey still lie ahead, and thus, the rest of this is complete speculation, following the structure of a Hero's Journey as a basic outline.
1. Refusal of the Return
This will be interesting, because so far it's been Sam who just wants to return to his normal life and Dean, who's asking him to stay. Usually, this also is the point where it looks like the journey is over, only the hero has "a bad feeling about this".
2. The Magic Flight
The baddies aren't dead yet! This is usually the last escape... hero escaping with the price, hero must overcome the last obstacle.
3. Rescue from Without
The hero's guide or soulmate (probably Dean) will bring them back to life, often via (self-)sacrifice. This can be sacrificing oneself, or sacrificing something that is very dear, for the good of the hero.
4. The Crossing of the Return Threshold
Back into the normal world again. Sometimes, the hero returns to his normal world, but he's always altered. And sometimes, he'll be so altered he can't return to his normal world.
5. Master of the Two Worlds
The hero will learn to master both worlds. Speculatively: Sam will be able to lead his normal life, but Sam also will be okay with hunting the supernatural. He will make peace with those two aspects.
6. Freedom to Live
The Happy End.