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A "walk-in" is where the original soul of a human leaves a person's body and another soul "walks in". Souls "walk-in" usually during an accident or trauma, where the person either regains consciousness as a "walk- in" and forgets somethings depending on the severity of the experience, or if they didn't lose consciousness the human will experience death and rebirth in one lifetime. One might need to relearn walking, drawing, writing, and upon doing so the new soul develops and integrates through the trauma. It is said that the original soul leaves when it has fulfilled what it came here to do, and the same body is used again for another soul to fulfill their purpose on earth.
Walk-ins in popular culture
The film Here Comes Mr. Jordan, and its sequels and remakes, are a take-off on the older, spiritualist version of the walk-in concept, although the term is never used. Heaven Can Wait, a 1978 remake of Here Comes Mr. Jordan, was also the inspiration for the television series Quantum Leap, in which Dr. Sam Beckett, involved in a time travel experiment gone awry, "leaps" into events that occurred during his lifetime, displacing another person. (Although most other characters perceive Sam as the person he replaced, this is not a true "walk-in", since his body is also involved.) In an attempt to return to his own time, he needs to change the future outcome of a situation for the better, after which he can make another leap. Beckett believes that his leaps are controlled by God or a God-like force.
Hawkgirl comics, the K-PAX series of books and films, and the Twilight Zone episode "The Last Rites of Jeff Myrtlebank" have all featured situations similar or identical to walk-in experiences, although the term "walk-in" is not used.
After the Death of Superman story cycle, a handful of new superheroes appeared, among them John Henry Irons, who called himself the "Man of Steel". He never claimed to be the real Superman, but Lois Lane speculated that if Superman were really dead, perhaps his soul had moved into Irons' body as a walk-in, and she used that word.
The term "Walk-ins" was used several times in The X-Files television series. In the episode ("Red Museum") of the second season, it was used to describe members of a (fictional) cult that believed in soul transference, in which enlightened spirits take possession of other people's bodies. In another episode ("Closure") of the seventh season, it was used to describe the spirits of dead children who come to convert living children from matter to energy (starlight) in order to save them from a horrible fate in life.
The TV series Ghost Whisperer (featuring Jennifer Love Hewitt) in the late 2000s used the term "step-in" when one character died, and their soul migrated into the body of an accident victim (season 4, episode 7, "Threshold").
Stephen King's The Dark Tower series of books, specifically Song of Susannah, featured characters known as "walk-ins". In these books, parallel universes are connected through "doorways" between worlds. These are actual doors created by magical means between parallel dimensions. "Walk-ins" are human or non-human beings who appear to have accidentally stumbled through to "our" world. They sometimes do not speak English, and sometimes speak languages unidentifiable to academics in "our" world. They often appear disoriented, confused, very ill, or suffering from severe physical defects.
In the cartoon series Danny Phantom, is about a boy who gains "ghost powers" by becoming half ghost. One of them is the ability to walk-in into other people, and sometimes ghosts, to control them temporarily. On the show this is referred to as "overshadowing" someone.
A similar "walk-in" concept, and variations thereof, was used by H.P. Lovecraft in several of his short stories. A good example of this can be found in the short story "Through the Gates of the Silver Key." In this story a man returns to earth in the body of an alien being. Other Lovecraftian tales portray a more conventional walk-in idea in which an alien being comes to earth and takes over the consciousness of a human. These tales are notable because the time frame in which they were written predates the above examples by many years. Lovecraft published his short stories in the early 1900s.