Donnie Wahlberg, who plays Danny Reagan on "Blue Bloods," has a real-life police mentor, James Nuciforo, a retired detective. Having spent 20 years on the force, Nuciforo brings authenticity to Wahlberg's character. Wahlberg has also received training from other cop roles he's played in the past. However, he admits that in previous roles, he was always concerned about being completely virtuous and likable. Playing Danny Reagan allows him to take risks and create dramatic tension in the show, making it stronger.