Warning: Due to the timeframe since past episodes have aired, spoilers up to Season 15 may be unmarked. If you have not watched up to Season 15, proceed with caution!
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- Eric Kripke's original idea for the show was more like equal parts X-Files and Scooby-Doo. It was originally an anthology series about two tabloid reporters who travel around in a van fighting demons. Then, after the show got reworked into what we now know and love, it was originally "Sal and Dean Harrison" travelling around in a '65 Mustang. The names were changed for legal reasons (as was their father's, which was originally Jack) and the car was changed because a friend suggested an Impala is a rougher, more aggressive-looking car (and because it's easier to fit a body in the trunk).
- Jensen Ackles originally auditioned for Sam and was the showrunners' favorite for the role until Jared Padalecki auditioned.
- Kripke had wanted Bruce Campbell (as in, Ash himself) to play the boys' father but Campbell was unavailable, and Padalecki's then-girlfriend Sandra McCoy auditioned for the role of Jessica. Jeffrey Dean Morgan only expected to play the 1983-year version of John due to his age and was surprised they had him come back to be the modern-day actor. McCoy still wanted to be on the show and went on to audition for the roles of Sarah in "Provenance" and Carmen in Season 2's "What Is and What Should Never Be" before landing the Season 3 "Bedtime Stories" incarnation of Dean's Crossroads Demon.
- An earlier draft of the script for "Pilot" included as a DVD extra on the Season 1 boxset depicted Sam and Dean as not originally raised hunters and having more years with their parents — Sam was 9, Dean 13 — before Mary was killed in a car accident caused by a demon, after which they were raised by an aunt and uncle instead of John. John was a lone hunter, Dean later found out the truth and joined him, and Sam thought they were both nuts until he was forced to see the supernatural himself. John was also killed in the pilot episode instead of Jessica. Considering the angst their upbringing caused them in the show and how it's turned out to influence their personalities and neuroses, it's bizarre to think of the boys living that apple-pie life as children without knowledge of the supernatural, and being new to hunting.
- Jess lived in that same script and the network wanted her to stay, but Eric thought that one of the main characters having a long-term, stay-at-home girlfriend wouldn't work for a roadshow about two brothers. (Apparently, the idea that she could come with them and become a hunter herself didn't occur to him.) Then the network suggested that he have her turn out to be a demon manipulating him, but he didn't use that either, preferring the idea of Mary and Jessica being murdered in the same manner as Book Ends. He did use a variation of the latter concept when he introduced Jessica's real killer in "The Devil You Know" — a college friend of Sam and Jessica's who became possessed by one of Azazel's minions and manipulated Sam into going back to hunting by tricking him into falling for Jess and then killing her in front of him.
- Demons were not planned to need to possess people; they were supposed to be part-corporeal, part-incorporeal creatures who manifested themselves on Earth, hence why John suggested in "Salvation" that Meg was "either" a demon or possessed by one. After having the demonic Monster of the Week possess different hosts in "Phantom Traveler" however, they eventually made it part of the demonic standard because it was supposedly more interesting. There were other parts of demon lore from "Phantom Traveler" that were dropped from the current versions — most glaringly, the word Christo was supposed to make demons reveal themselves (and still survives somewhat in fandom) and would've come in handy at multiple points (perhaps most of all in "Celebrating the Life of Asa Fox"), but was not used further by the writers because it killed the suspense of who was a demon. An episode in a later season did retroactively explain it as only working on less powerful demons.
- Missouri was planned to be a recurring character and she was supposed to have Bobby's role in "Devil's Trap", but Loretta Devine declined because she wanted to focus on her Broadway career. Bobby was created in her stead and Missouri would not appear again until Season 13 to introduce her (previously unmentioned) granddaughter Patience for the "Wayward Sisters" spinoff; while she was as awesome as we remembered, she was also killed off early in the episode for the sake of drama and motivation for the heroes, leaving us with just two episodes total of Missouri.
- The writers initially had trouble deciding which Winchester would be possessed by Azazel in "Devil's Trap" and toyed with the idea of Dean or Sam being possessed instead.
- Jo, originally named Alex, was meant to be Dean's Love Interest, the writers' first attempt at a recurring one for either brother. Their efforts can be seen in the first few episodes of the season. Unprecedented negative reaction made them keep the brothers away from the Roadhouse until they wrote Jo out indefinitely in "No Exit", and they were reluctant to try it again for years because of how thoroughly their attempt with Jo was disliked by the fans, preferring to write well-received female antagonists like Meg instead and directly resulting in the creation of Ruby in Season 3. The writers also added in the idea that Dean only saw Jo as a little sister figure instead of a love interest because that's how she came off as to many fans (she is younger than Sam, after all), which the writers had ended up agreeing with. They brought the character back in "Born Under a Bad Sign" to quash the idea of a romance between Jo and Dean, though the ship wasn't entirely sunk: Kripke and Alona Tal got the last laugh in "Abandon All Hope...", with Jo brought back in Season 5 for a send-off in which Dean hit on her and then later gave her a heartfelt kiss during her Heroic Sacrifice. As Jo Took a Level in Badass for her return, fans were more forgiving of it.
- The special children plot was supposed to continue into at least Season 3, but the writers came to dislike the idea because the psychic kids just weren't interesting antagonists and Kripke disliked how dense and confusing they made the plot. To resolve it, they gathered the psychics up in the Season 2 two-parter finale to kill them all off except Sam (well, including Sam actually, but you know how that went). This was justified In-Universe by Azazel going from claiming they would be his soldiers in a war, to claiming he only needed one soldier (to lead his demon army) and he wanted them to kill each other off to find out who was strongest. It worked in this case since demons lie and are very manipulative, so it didn't feel out-of-character for Azazel to turn out to have been jerking their chain about his real plan. In hindsight, it also makes sense that Azazel would have them all kill each other, in order to determine which one was fit to be Lucifer's vessel.
- Ruby was originally written with Kristen Bell in mind for the role, but she declined. Lauren Cohan auditioned for Ruby and Katie Cassidy auditioned for Bela before they got the parts they ended up playing.
- Bela was not designed as a series regular. She was created for "Bad Day at Black Rock" and the writers liked the idea of her so much that they just picked her to be the second female regular when the CW wanted one more in addition to Ruby. That she wasn't planned to add to the season's plot was evident by the way she waltzed in and out of random episodes, duping the Winchesters almost every time because the writers enjoyed having a female antagonist so irritate the boys; unlike Ruby, she added virtually nothing to the overarching plot until she stole the Colt and was revealed to be trying to escape her own Deal with the Devil (mirroring Dean) by killing the Winchesters on the Big Bad's orders. Kripke might have come up with her backstory and motives once she was upgraded to series regular, as he offered to tell Cohan about it before filming for the season, but the failure to tie her to the plot in an interesting and organic way sooner resulted in Bela becoming so loathed that the writers had to kill her off the same episode they revealed it. And as many found the concept of her character interesting and even sympathized with the final depiction of the character, had Bela been used more sparingly (as originally planned) or simply used or written differently, she could have avoided the fans' ire and survived the season.
- Early plans for Season 3 had Gordon finding out about Sam's actions in "Born Under a Bad Sign" while possessed by Meg (though it's unclear if he would have known about that part, or even cared if he had) and rounding up a posse of fellow hunters to hunt Sam down. Because Sterling K. Brown was contracted to Army Wives and they'd only let him appear for two more episodes, the Supernatural writers were forced to heavily condense Gordon's storyline by having him convince one hunter to help him break out from prison in two scenes in one episode, and find and get killed by Sam after getting turned into a vampire in the next.
- An apparition of John Winchester was originally going to confront Dean in the climax of "Dream a Little Dream of Me". Unfortunately, Jeffrey Dean Morgan wasn't available at the time, so they replaced him with Jensen Ackles and did the "I'm my own worst nightmare" thing instead, making his dream even more depressing; canceling out his assertion of self-esteem while he was saying it.
- Jason Voorhees (and not a Captain Ersatz) was also going to appear in "Dream a Little Dream of Me". They went as far as securing the rights to use Jason when a few days before filming started, the studio they got the rights from realized they didn't actually own the rights and the writers had to come up with a new scene in which Sam has an Erotic Dream about Bela to replace what would've been Jason's airtime.note
- The biggest of them all: had the 2007-2008 Writers Guild of America strike never happened, Season 3 would've gotten the full 22 episodes, Sam would have agreed to let Ruby teach him how to use his demon powers, and he would've saved Dean from his deal (as originally intended by the writers). This would've given us major differences on the characters and the lore.
- "Mystery Spot" and "Jus in Bello" were the last episodes created with the original plan in mind, and the writer's strike allowed for only four more episodes to wrap up the season. Because they couldn't develop Sam's powers in that time, they decided to shock the audience by having Dean actually killed and sent to Hell. This resulted in Dean's trauma over Hell, a darker character turn, and the introduction of fan-favorite villain, the Creepy Awesome Torture Technician Alastair as Dean's personal tormentor. How a darkly empowered Sam would've played out in these circumstances is also unknown, how he would've been characterized (tapping into his powers in desperation to save Dean, instead of when he's grieving and angry), whether he would've had the same demon blood addiction storyline, and whether he would've saved Dean by killing Lilith early.
- If Sam had saved Dean, the writers wouldn't have needed to come up with a way of getting Dean out of Hell and more than likely the entire angel mythology never would have come into the show, as Kripke had repeatedly shot down the concept before because he preferred the idea of God working through hunters. That would've meant no Cas, the Breakout Character of the show, which would have resulted in a very different Supernatural than we know today, and Misha Collins might not have joined the cast at all (or if he did, he would've had to be playing a different character, likely far different from Castiel's unique character). As angels were not originally planned but Lucifer was according to Kripke's five-year plan, it also isn't known how that character would've been implemented.
- While unconfirmed, it is likely Lilith's original Evil Plan was also changed because of the writer's strike and plot changes, since there is no mention of seal-breaking in Season 3 and Lilith and her minions never even mention Lucifer. While Ruby was said to always have been planned to be a Hidden Agenda Villain and likely always meant to be trying to carry on Azazel's plan of corrupting Sam, Lilith's multiple attempts to have Sam killed across the season come across as a genuine desire to see him dead and the audience's knowledge that Sam was initially supposed to unlock his demon powers that season and be strong enough to save Dean makes her wariness of him as a rival (as stated by Ruby) look like a straightforward reason to want him eliminated. All things considered, Lilith and Ruby were almost certainly not planned to be working together at that point, but rather meant to be rival factions, which would've fit the writer's original plan for Season 3 having chaos and multiple factions in the demon world.
- The writer's strike also prevented Ellen's return until Season 5; she was supposed to appear in at least two episodes in Season 3, but the strike forced the first episode she would've returned in to be dropped and Samantha Ferris rejected an offer to return in the finale instead, in part because she feared her character would be killed off. Whether she would've is unknown, but they apparently didn't reassure Ferris otherwise. Since Jo was not mentioned as supposed to return as well alongside her mom and was still not fondly remembered, Ellen might have been brought back after losing the Roadhouse and still not reunited with her MIA daughter. Both characters missed Season 3 and Season 4 entirely, reuniting offscreen and returning for two episodes in Season 5 as experienced hunting partners.
- Katie Cassidy's departure from the role of Ruby was not planned; according to Kripke it was budget cuts, according to Cassidy she left on her own because the studio didn't know what they wanted to do with Ruby. This happened right as fans were warming up to Cassidy's portrayal of Ruby, and they were even more displeased with her successor than they were with Cassidy herself, as Genevieve Cortese was instructed to play the character completely differently and regarded as less skilled than Cassidy, and it was difficult to see the two incarnations as the same character. Had Cassidy remained in the role and continued to play Ruby as aggressive and action-oriented as she had in Season 3, or even had Cortese been instructed to retain more of Cassidy's portrayal of the character, the Season 4 incarnation would likely have received less backlash. (Though it worked out great for Jared and Genevieve Padalecki.)
- Castiel was originally conceived of being John Constantine himself. Adding him into the show turned out to be too complicated, so he became a member of the Trenchcoat Brigade. Would've been interesting to see how that would've shaken out, since Constantine later got a (short-lived) show and guest-starred on another CW show. Presumably this plan was before he was decided to be an angel.
- The writers wanted to utilize The Nth Doctor to the fullest by having Ruby portrayed by a revolving door of different actresses throughout the season because of the unique opportunity having a Body Surfing recurring character afforded them. They changed their minds because Kripke liked Genevieve Cortese's performance in the role. Lucky he did because after the first episode aired implying Sam had had sex with Ruby, there was Fridge Horror from fans alarmed by the implication of Sam having sex with a host body who was either unconscious during sex or awake, helpless, and unable to give consent. Sera Gamble wrote a flashback in "I Know What You Did Last Summer" showing Sam refusing to associate with Ruby in a previous host because she was using a living person and Ruby possessing a braindead, flat-lined coma patient whose soul had already moved on before they eventually did the deed. Had they gone ahead with their original plan and Ruby used as many different hosts as they'd wanted, it would have broke the audience's Willing Suspension of Disbelief that Ruby kept happening across that many pretty, just flat-lined coma patients to possess and appease Sam's morals.
- Castiel was only supposed to last for six episodes in season 4 before being killed off by Alastair in "Heaven and Hell", reportedly (according to popular fan rumor) leaving Anna to fill in as the Winchesters' angelic adviser and Dean's eventual end-game. Positive fan reaction not only brought him more screentime and A Day in the Limelight that season, but the character was promoted to the main cast the following season and has remained a major character ever since.
- In "Are You There, God? It's Me, Dean Winchester", where ghosts blame the boys for their deaths, Sera Gamble tried to get Jessica Moore instead of one-off character Ronald from "Nightshifter" but it fell through because Adrianne Palicki wasn't available. Thank God she wasn't though, as her angrily blaming Sam for her death would have probably reduced everyone to tears.
- As disclosed in the official companion guide, Eric Kripke had intended for Ruby to die in the fourth season finale but when watching Genevieve Cortese's performance during filming that he started having second thoughts, questioning why he was writing Ruby out so soon after dropping a game-changing bombshell about her character and realizing it would've made more sense to have her continue into Season 5. Because they were already filming however, he figured they didn't have time to change it and she died as scripted, making this a case of They Wasted a Perfectly Good Character from the writers' perspective. Not that fans minded.
- The writers wanted to bring Nicki Aycox back to play Meg in Season 5, but couldn't figure out a good way to justify why she'd come back in that body (which, at that point, had been dead and buried for four years in-universe) and so decided that they had to recast. If Aycox had come back instead, it's possible that Meg wouldn't have been killed off, as Rachel Miner (who was ultimately cast) was seemingly unable to continue playing her due to her illness, and the character was killed off rather than recast.
- There were fan rumors that Anna would have at least two appearances in Season 5 and she has also been confirmed as originally meant to make her return much earlier (in "Good God Y'all!"), but her part was cut for time and her return pushed back. She ended up appearing only once and was Killed Off for Real after turning on the Winchesters. Had she appeared as early as intended, it is unclear if she still would've been trying to kill Sam and perhaps become a recurring threat for the season, or remained an ally with her role in "The Song Remains the Same" being given to another angel character.
- Sam being Lucifer's destined vessel was to be saved as a big reveal for "The End", but the writers figured the fans would guess it anyway and revealed it in "Free to Be You and Me" instead. The Bad Future depicted in "The End" was also supposed to be a Stable Time Loop with the older Dean remembering the episode from the present Dean's perspective and revealing he had gone through it himself, making his desperate pleas for the present Dean to just say yes to Michael, and his realization Dean wouldn't because he hadn't, even more heartbreaking, as "In the Beginning" had established that the past can't be changed. Robert Singer thought it was too complicated for the audience to understand and they removed the explicit details of this from the story, leaving it unclear if End-verse was a possible future, an alternate universe's future, or a fake world cooked up by Zachariah. To make it even more depressing, in Ben Edlund's vision Castiel was to have been Driven to Madness by failing to save the world after he'd given everything, and just sit there killing and resurrecting a cockroach with a poke of his fingers. Forever. They've done some dark stuff before and since, but even the other writers were too unsettled by the idea to go through with it.
- Season 5 was expected to be the final season and was written accordingly by bringing back and killing off a number of characters who still needed plot resolution, until the series was renewed late in the season. Had the series been renewed earlier, Jo, Ellen, Anna, and Zachariah may not have been killed off when they were.
- In the DVD Commentary for the episode, Eric Kripke mentioned that he intended to permanently kill Castiel off at his older brother Lucifer's hands in the Season 5 finale, but Sera Gamble talked him out of it. Whether she meant to permanently kill Castiel off at the start of Season 7 has been debated, as Misha Collins and other crew associated with the show at the time gave every indication that Cas was indeed Killed Off for Real, but given Castiel's dying declaration to make up for his actions to Dean, it seems likelier Gamble had intended to bring him back and the crew was just covering it up.
- It was revealed that had the show concluded with the fifth season as was the original plan, the ending of Dean with his new family would have been the same, but the shot of Sam watching him from outside the house provided as a Sequel Hook when the show got renewed would have been eliminated.
- Lenore from "Bloodlust" was supposed to have Meg's role in "Caged Heat"—teaming up with the Winchesters to find and kill Crowley, which makes sense, considering he was hunting her kind down for Purgatory—but Amber Benson couldn't appear, so her part had to be rewritten and Meg and her merry gang of Lucifer loyalists appeared instead. Had Lenore been able to fill the role in the episode as originally planned, Meg presumably would not have had reason to ally with the Winchesters and would've remained a full-on Love to Hate villain.
- Additionally, Meg was intended to have a larger role in seasons 6 and 7, which was curtailed when Rachel Miner pulled out due to medical issues.
- John as played by Jeffrey Dean Morgan was at one point considered to be brought back in the Alternate Universe instead of Ellen in "My Heart Will Go On". Concerns that they wouldn't be able to secure the return of the actor played a part in it not going through.
- Season 6 was originally planned to be very different from what ended up onscreen. The realization that Sam had lost his soul and the brothers' attempts to get it back were planned to be the main arc, but fan backlash forced the writers to conclude it halfway through the season, and Castiel being the Big Bad was only thought of at the last minute as a replacement.
- Amelia was supposed to return in the back half of season 8. The role filled by Sarah Blake was originally meant for her. By that point however, Amelia had become so despised by the fans that the writers were afraid they'd be cheering for Crowley to kill her. So instead they decided to bring back Sarah Blake, a minor but relatively well-liked character from season 1, and kill her off instead. Ironically, fan hatred for Amelia ultimately made her one of the few Love Interests to survive Sam's Cartwright Curse.
- Had the Covid-19 pandemic not curtailed production, the series finale would have had an epic moment: In Heaven, Dean meets scores of past characters (rumored to have included anyone from John and Mary to Jo and Ellen) in a roadhouse where the band Kansas would have appeared as themselves to play "Wayward Son."