It's not quite as simple as planting the suggestion to do something. It's changing your entire perspective. This arm is now my arm. This skin is now my skin. These lungs are now mine. And if you don't quiet the host it's terrifying for either party. [Aaaand he pours himself another glass.]

Resistance to and recognition of suggestion and being quieted for manipulation are the two things that are going to be most important to learn. After that, most entities would merely pass him by for an easier, less cognizant mark.