Beckett: Siri tell Castle to stop starring while I am doing paper work.
Siri: I am sorry did you mean "I love when you look at me like let's have beautiful children?"
Castle: Siri tell Beckett I will not help out with the paper work
Siri: I am sorry did you mean "Will you marry me?"
Beckett: Siri what should I wear today
Siri: I suggest you wear your purple top. It is Castle's favorite
Castle: Siri list all coffee shops in the area
Siri: Here are the one's closet to you. I suggest you pick of a pastry as well Beckett will appreciate it.
Siri: I suggest both of you stop denying your love. (Siri would be the ultimate shipper)
(They do have Siri, or at least Castle does on his phone. Saw it one of the last episodes.)