People who pass away before their karmically allotted time (akalmrityu), who commit suicides, who died violent deaths or in accidents, remain in a state of utter confusion and disorientation, without sometimes realizing that they have crossed over. It also happens to people whose last rites are not performed and are not released in the spirit world. Unable to satisfy the urges of their bodies, the desires they crave for- food, sex etc, since they no more exist in the bodies, they become frustrated and angry and look for an opportunity to enter back the physical world to enjoy the pleasures of the senses. 

They may lure living people by their knowledge of past, present and future and possess their minds if they grant access to their minds easily. They may grant them favors and this usually happens to people who give in to temptations without thinking of their consequences, everything comes at a price. 

A friend of mine was once positioned in the house of a media baron, who was totally engrossed in the material world and could think of nothing but riches, pleasures and more business. As evenings would progress, she would see dark hooded shadows looming over and around this lady, yet this lady enjoyed all the pleasures of the senses including varieties of cuisines, bodily pleasure and indulgences even at the age of 72. In specific shmashansadhnas, one can gain favour from the kind spirits who look for opportunities to enter the physical world for the fulfillment of their own desires and they grant you pleasures they themselves seek. But they never leave you, you become their slave and when the time comes to leave your body, you go with them, they take you to the nether world where you become a pishach like them and pay for the pleasures you were granted by them.