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, either Castle wore her down or she is taking 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.