All the usual disclaimers apply: I don't own Castle, never will, and certainly have no chance of ever profiting from it.
Yes
It's become a game between them, and no one else is allowed to play. He casually tosses out an innuendo, and she shoots him down. No harm, no foul, no offense taken.
That changes when they begin investigating a corpse in a pirate costume. Castle offers to show her his Jolly Roger, and she instantly thinks, At full mast? A hot flush spreads over her face, and her hands clench convulsively. She bites her lip hard enough to taste blood and mumbles a weak rejoinder about keelhauling.
Her mouth can deny him all it wants to; her body's already decided.