When Castle is first looking at the neighbors with the binoculars. He sees a man talking to a woman with a laptop and says, "Hmmm, must be writers." The man and woman appear to be creator/writer Andrew Marlowe and writer Terri Miller.
The trivia items below may give away important plot points.
Aired on April Fool's Day. At the end of the episode, the "murder" that Castle believes he witnessed and the subsequent investigation are revealed to have been part of an elaborate prank Beckett set up as a birthday surprise.