It would take the form of another human being close to me, a friend or a family member. That is it would invade this person, without him knowing it happened.
How sure am I of the above? Not at all, but that's the best answer I can think of at the spur of the moment!
There are 2 issues involved.
1) Demonic Influence.
2) Demonic Possession.
Believers are indwelt by God the Holy Spirit, and for this reason cannot be demonically possessed. They may be heavily tempted and influenced by demonic influence.
Demons, aka fallen angels, may be an entire range of creatures. They aren’t all the types depicted in movies, though those are based upon reported events.
For a believer who is continually attacked, and is victorious in rejecting temptation, they frequently are then attacked by persons close to them who are in turn tempted by demonic influence or possibly even possession.
Possession doesn’t mean the person is acting nutty and violently. They might be very worldly, calm, rational, and keenly observant.
One observation,...when a person is in the right place, at the right time, doing the right thing, in the right manner, they frequently will be attacked.