- "Feed the birds" - A type of handshake done in gratitude or to show that one person cares for the other. It is done by putting the tips of all the fingers on one hand together and making them meet with the tips of the fingers of another person. One person points their fingers down and the other points theirs up and both tap their fingers together. This action appears in Almost Famous between Castle and his daughter Alexis, between Castle and Detective Javier Esposito in both Vampire Weekend and Close Encounters of the Murderous Kind, and between Castle and Detective Kevin Ryan in Castle, P.I..
- "Double rainbow all the way" - an expression Nathan borrowed from a video posted to youtube.com in which a tourist site seeing at at a national park sees two rainbows side-by-side and exclaims "double rainbow all the way". The video has gone viral, having been viewed about 26 million times as of mid-March, 2011. Nathan uses it to mean something is amazingly cool. He tweets it regularly and sometimes abbreviates it as DRATW. He alludes to it in He’s Dead, She’s Dead, when he asks Beckett if she believes in unexplained phenomena, such as double rainbows.
- "Bam, said the lady" - another expression used by Nathan, but to mean "there you have it" or "bingo". He also tweets this expression regularly and sometimes abbreviates it as BSTL. It is used in To Love and Die in LA, among other episodes. Nathan has also used varied versions, such as "Blam, said the lady" in Last Call, "Zap, said the lady" in The Final Frontier, and "Bam, said the costume lady" in Pretty Dead.
- youtube.com video Posted 8 January 2010