Favorite Castle Line: Beckett's Clone

Beckett allows an actress, Natalie Rhodes, to visit the precinct and follow her around--so Rhodes can get in touch with the character, Nikki Heat. 

Since this is the type of behavior that Beckett usually protests, she is likely trying to take him down a peg or two. 

The decision backfires when the actress proves to be a "method" actress! She adopts Beckett's mannerisms and dress to an obsessive degree. 

Hence, Ryan's line delivered in Seamus Deaver's deadpan way: "We should have a code word, so we all know which Beckett to kill when her clone army attacks." 

Beckett responds, "Unless we make a preemptive strike."

The episode is extremely well-written. In the end, despite her obsessive behavior, the actress and the precinct part on good terms. Unfortunately, as we learn later, personal matters prevent her from taking the role.