The Wikipedia article linked above contains a section regarding that. Although some "marks" had previously worked as actors or extras, they had not been hired

The Wikipedia article linked above contains a section regarding that. Although some "marks" had previously worked as actors or extras, they had not been hired for this purpose, and the production company had recruited participants through websites "popular with people keen to appear on television" Personally, this didn't bother me since I was more interested in finding out how the cons worked.