A shill, also called a plant or a stooge, is a person who publicly helps a person or organization without disclosing that they have a close relationship with the person or organization.
"Shill" typically refers to someone who purposely gives onlookers the impression that they are an enthusiastic independent customer of a seller (or marketer of ideas) for whom they are secretly working. The person or group who hires the shill is using crowd psychology to encourage other onlookers or audience members to purchase the goods or services (or accept the ideas being marketed). Shills are often employed by professional marketing campaigns. "Plant" and "stooge" more commonly refer to any person who is secretly in league with another person or organization while pretending to be neutral or actually a part of the organization he is planted in, such as a magician's audience, a political party, or an intelligence organization (see double agent).