You have to 'prepare' that person somehow in order to do it.
That's not entirely true, otherwise you would never be able to use a possessing spirit to grab some random guard you stumble across and possess him.
As per Street Grimoire pg 197, the possessing spirit rolls an opposed roll of Force x 2 vs the target's Willpower + Intuition for living or against Object resistance if a device/object.
Preparing the vessel ahead of time grants the spirit a +6 dice boost to their side of the roll and is usually much preferred due to the bonus dice, especially if you are trying to possess some high tech item because of the Object resistance or a tougher Mook.
So you can still just possess someone randomly off the street without the prepping, it's just easier if you are using a prepared vessel.
Just keep in mind this is on par with Mind Control for NPC Horror Factor and they tend to get nervous about being around folk who hijack bodies regularly, so many go down the prepared vessel route since that mitigates it a bit by using someone who agrees to the possession ahead of time.
So it is not uncommon for the possessing mage to have another team member or a flunkie (the gun caddy or a ganger the team regularly works with for example) prepped to receive the spirit since the boost in stats can be handy in a pinch.