A walk-in is a new-age concept of a person whose original soul has departed his or her body and has been replaced with a new, generally more advanced, soul.Ruth Montgomery popularized the concept in her 1979 book, Strangers Among Us.
Believers maintain that it is possible for the original soul of a human to leave a person's body and for another soul to "walk in". Souls are said to "walk-in" during a period of intense personal problems on the part of the departing soul, or during or because of an accident or trauma. The individual retains the memories of the original personality, but their personality and abilities change and become oriented towards helping humanity. Incarnating into a fully grown body allows the more advanced soul to carry out its mission without having to go through the two decades of maturation that humans need to reach adulthood.
The Hawkgirl comics, the K-PAX series of books and film, 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.
In 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 X-Files episode "Red Museum" discusses walk-ins, described by Mulder as enlightened spirits who have taken possession of the bodies of people who have lost hope and who want to leave their life. The concept is returned to in the episodes "Sein Und Zeit" and "Closure".
In the TV series Ghost Whisperer, the season 4 episode "Threshold" used the term "step-in" when the soul of one of the series' main characters, who had died in the previous episode, enters the body of a man who dies in an unrelated accident.
Stephen King speaks of "walk-ins" several times in books 6 and 7 of The Dark Tower novels, but King's walk-ins are usually physical travellers, or - when they possess another's body - are more guests, sharing the body with the original mind as strangers. John Callum mentions them in The Dark Tower VI: Song of Susannah. The term is also used in the CODA section of this book.