'Demonic Possession' and 'Walk-In'

The first term refers to the partial or complete takeover of a person by either a malevolent spirit or a corporeal being, which has the ability to leave it's body to enter another one for it's own interests. The term 'demonic' just referring to the selfish forced nature of the process and not to a particular kind of being. It can be the result of a contract with such an entity or merely very unconscious living, drug and alcohol abuse or another state of low vibration in which the will of an individual - in particular the will to live - is considerably reduced.

'Walk-In' in contrast is the benevolent version of a body takeover. It is entirely voluntary and based on a soul contract. It occours frequently, when a soul feels that it's current life topic is too tough and offers it's body to another. A different reason can be, that an advanced soul wants to incarnate on earth but wants to skip the cumbersome process of being a child and another soul volunteers for the assignment. It's outer form is generally a traumatic event like an accident or severe illness close to death and a miraculous recovery with the recovered individual showing a marked difference in personality.