The compulsion to kill is a hard one to shake ... especially when you work for the police. And in the upcoming third season, Dexter will have some help. Valerie Cruz has signed up for the third season of the Showtime serial killer series. Taking on the role of Mrs. Prado, Cruz will be paired with the recently cast Assistant District Attorney Miguel Prado (Jimmy Smits) and the duo will take Dexter in new directions. Smits will be appearing in 10 episodes from the third season and it looks like Cruz will be having a similar length of involvement. Dexter is forced to partner with Miguel Prado in order to track down a killer that has a direct impact on the lives of both characters. “Dexter” ended its second season run with a ratings high for Showtime when “The British Invasion,” managed to draw 1.4 million viewers in its 9.pm. broadcast making it the most-watched audience for a Showtime series. The episodes encore presentation also snagged an additional 382,000 viewers. “Dexter” returns to Showtime in the fall.