Part of the Casswiki article series New Age and Channeling and channeled material
“Walk in” is a generic New Age term meaning a spirit that takes over the life of a human sometime in the course of his/her life. It is an ancient concept first described in Hinduism whose modern name originated in the Spiritualist faith and was later popularized by the New Age movements.
The difference between this and the idea of possession is that this is supposedly a deal done in free will between the former and new occupant of the body. This is sort of an alternative to suicide from the viewpoint of the original occupant and a quick ticket to Earth from the viewpoint of the new occupant.