From "The Dead Pool", interrogating the Byelorussian: 

"I was expecting 'moose and squirrel'."

From "Close Encounters of the Murderous Kind": 

When Castle checks on his phone as to what is available off New Jersey Turnpike Exit 9, the second attraction is 'Lilliputin State Park".  A reference to Lilliput (the imaginary kingdom in Gulliver's Travels).  It is also interesting that there is a 'lilliput mussel' which has invaded New Jersey recently,  and has been the subject of some ecological mapping. 

(The first attraction is the Assanti Arts Centre, which I assume has some reference to the kingdom of Ashanti in Ghana. But that may not be the correct reference). 

In "Cuffed" Nathan suggests that they cut off a hand to get out of the handcuffs — hers because "it's smaller" — and he makes obvious reference to the recent "127 hours" film, but the original reference is to the first man-and-woman-handcuffed-together scene ever, in "The 39 Steps" — Madeleine Carroll is able to slip out of the handcuffs (it takes her awhile) because of her small wrist. 

PrettyFloralBonnet said:

Toronto said:

In "Cuffed" Nathan suggests that they cut off a hand to get out of the handcuffs — hers because "it's smaller" — and he makes obvious reference to the recent "127 hours" film, but the original reference is to the first man-and-woman-handcuffed-together scene ever, in "The 39 Steps" — Madeleine Carroll is able to slip out of the handcuffs (it takes her awhile) because of her small wrist. 

A bit OT, but have you seen the BBC remake of The 39 Steps with Rupert Penry-Jones and Lydia Leonard?  The way they played the 2 main characters of Richard Hannay and Victoria reminded me of Castle and Beckett the first time I watched it.  The fine art of bicker, banter and eye sex.

Hi, I did see it, and really enjoyed it.  Some people I know didn't like it, but I thought the two leads were brilliant together.  It was obvious that the ending was designed for there to be a sequel, but I guess that never happened.