Castle (Nathan Fillion) and Beckett (Stana Katic) will be in a flashback '40s episode, which will see Castle as a "Sam Spade type" and Beckett as "a gangster's moll", according to TV Guide.
"We flashback to the 1940s and all the Castle gang take on various stereotypes of the '40s, of the '40s film noir kind of genre," said Katic at the SAG foundation panel.
"Everybody's already looking forward to getting their accents and film noir on."
Exec Andrew Marlowe revealed that the show will also have a Dancing with the Stars-themed episode in the spring to coincide with the ABC dance show's return.
Castle airs at 10/9c on ABC.
> Smallville star Justin Hartley for Castle role
> Hilarie Burton to guest star in Castle