In his 1990 book "An Exorcist Tells His Story," Father Gabriele Amorth, cheif exorcist of Rome says that possession doesn't happen overnight. It's a process, and it always requires an open door: playing with Ouija boards, attending or conducting séances, even going on a ghost hunt with friends. No matter how innocent one's intentions, dark spirits can take advantage of such opportunities.
Fr. Amorth identified four stages of demonic activity:
1. Infestations
This is "haunted house" type stuff: footsteps, voices, apparitions, furniture or other objects moving without human agency, odors with no discernable source. Rather than directly affecting people, infestations affect only property, objects, or even animals.
2. Oppression
Activity steps up with physical attacks, sleep disturbances including regular nightmares, frequent and severe illnesses, major depression or anxiety, severe financial or employment problems, and relationship troubles. While these things happen in the normal course of life, all of them happening at once or in rapid succession could be a sign of demonic presence.
3. Obsession
As the name implies, at this stage the affliced person has a hard time functioning, being constantly preocuupied with thoughts of the demonic activity commandeering his or her life, and frequently with thoughts of suicide as well. Sleep becomes nearly impossible.
4. Possession
Contrary to popular belief, possession is not demons entering a person's body and taking over his or her soul. A person's free will is never removed, only severely compromised. In possession, a person is so physically, emotionally, mentally, and spiritually broken down by going through the other three stages that demonic spirits are able to seize occasional control over that person's actions.
Telltale signs of possession include superhuman strength, speaking in a language the victim doesn't know, inordinate aversion to holy objects, knowledge of events or facts the victim could not possibly know, and changes in facial features.