Warning: spoilers ahead for The Winchesters episode 8.
Jensen Ackles' Dean Winchester has made another appearance in The Winchesters, and his photographic cameo leaves big Supernatural questions to ponder. Jensen Ackles has been attached to The Winchesters as a producer since the Supernatural spinoff prequel was first announced, but the plot thickened when the actor was confirmed for an onscreen presence also. Dean, alongside his trusty Impala, appeared in The Winchesters' series premiere, narrating the story of how his parents met and then cruising into the distance. Exactly how Dean could be physically present and investigating John and Mary's past remained unclear due to his death in the Supernatural series finale.
Dean's contributions to The Winchesters episodes 2-7 have been solely as a narrator's unattached voice, but that all changes with episode 8, "Hang On To Your Life." Reunited with her father, Samuel, The Winchesters' young Mary Campbell and John Winchester rifle through recon photographs taken by the recently returned hunter. Among them is a picture of Dean Winchester hopping into his Impala. To complicate matters further, John identifies this man as the person who handed him Henry's letter - and thereby kicking off The Winchesters' entire narrative - after he returned from Vietnam.
How Dean Is In The Winchesters' Past Timeline
The Winchesters' big Dean Winchester twist confirms Jensen Ackles' character is physically in the prequel timeline - a detail that was not clear during his premiere cameo. Somehow, Dean has traveled back to the 1970s and deliberately interacted with his father. The Winchesters has not yet confirmed the exact mechanics by which Dean achieved this feat, but Supernatural mythology presents two major possibilities.
The obvious option is time travel magic - a long-established concept from Supernatural, and a trick Sam, Dean, Henry and John have all utilized during their hunting careers. At some point during the original Supernatural timeline, perhaps Dean Winchester made yet another trip into the past, unseen by audiences, where he meddled with history and hung around in the 1970s long enough to get photographed by his future grandfather. Plausible though this theory may be, it seems unlikely that Dean had time to sneak into the past without Sam's knowledge, and that this huge event was somehow glossed over by Supernatural.
A more likely option is that Dean Winchester's role in The Winchesters is happening after his Supernatural death. Although being in Heaven should prevent Dean from exploring the 1970s as a living man, the eldest Winchester brother is well-acquainted with the boss upstairs, Jack. Dean Winchester has perhaps been afforded special permission to take a vacation from the afterlife and travel to 1972. This also accounts for the presence of Dean's famous Impala from Supernatural, which John is yet to even purchase in The Winchesters' timeline.
What Dean Giving John The Letter Means
Now that The Winchesters has confirmed Dean Winchester was the mystery man who gave John a letter from his absentee father, a whole range of story possibilities present themselves. One potential motivation, and, indeed, the most obvious, is that Dean simply wished to spare his father a life of unjust pain. Because Henry Winchester disappeared without explanation, John grew up believing his father abandoned him, and this had a profoundly tragic impact upon both his life and parenting skills.
In Supernatural, Sam and Dean discovered Henry was forced into leaving his timeline by the Knight of Hell Abaddon, but John was long dead by this point. With death potentially releasing him from the shackles of a single timeline, the older Dean Winchester perhaps wants to right that wrong by handing teenage John a letter that will lead him toward the truth. By reuniting with Henry's ghost in The Winchesters episode 7 and sharing a heartfelt final conversation, Drake Rodger's John Winchester has already found the closure Jeffrey Dean Morgan in Supernatural did not.
Another explanation for Dean's history-meddling is the Akrida. The Winchesters has confirmed the Akrida are from another dimension, which means they don't necessarily adhere to temporal laws. For example, the Akrida attacking 1972 in The Winchesters may not mean the John and Mary from Supernatural lived that same experience. The Akrida could be changing history just as much as Dean Winchester is. Dean perhaps knows the only way to defeat the Akrida - and protect his universe's future - is bringing John and Mary Winchester together earlier than fate intended. Dean's presence in photographs taken by Samuel Campbell while he was investigating the Akrida is further evidence of this connection.
Dean Winchester's Return May Confirm A Massive Prequel Theory
The Winchesters' very premise ruffled Supernatural feathers, since John Winchester should not begin hunting until Mary's death much further down the timeline. From there, numerous other inconsistencies between shows have sprouted. One theory to explain these canon plot holes was that John receiving the letter had triggered a deviation in the Supernatural timeline, since the serious changes only began to occur after that point, and it was the letter that ultimately resulted in John encountering Mary and learning about the paranormal. The Winchesters episode 8's revelation that Dean Winchester handed John the letter from Henry would appear to confirm that theory.
Rather than breaking canon or taking place in a parallel Supernatural universe, therefore, it seems The Winchesters is actually an alternate history caused by Dean Winchester - and possibly the Akrida. Everything before John's return from Vietnam played out the same as in Supernatural, but Dean's arrival and the curveball of Henry's letter have drastically altered John's intended path, creating a divergence between Supernatural and The Winchesters that accounts for almost every continuity discrepancy.
Dean Has To Return Again In The Winchesters Now, Right?
If the extent to which Jensen Ackles would feature in The Winchesters was unclear after his premiere cameo, Dean's episode 8 appearance leaves little doubt of his future importance. Dean's lack of contact with John and Mary since The Winchesters began suggests he likely intended to quietly hand over the letter and slip away to attend to other business, but now that Samuel Campbell has snapped him, Dean will surely become embroiled in the main Winchesters Akrida storyline. Dean must then hide his true identity from his young parents, which may prove difficult considering his unusually strong emotional investment in the health of John and Henry's relationship.
However Dean came to be in 1972, there is surely a limit to his involvement. If Dean used time-travel, he must avoid affecting history as much as possible in order to not "Back To The Future" himself. If Dean came from Heaven with Jack's permission, on the other hand, he is perhaps bound by certain rules of non-interference. Having said that, The Winchesters is heavily hinting that Dean will eventually encounter John and Mary's younger selves in a future episode, and that a three-way Supernatural generational team-up will form against the Akrida.