It's already been spoiled that Castle is planning a Comic Con themed episode later this year. In preparation, Star Trek: Deep Space Nine alum, Armin Shimerman has been tapped to guest star as Benjamin Donnely. Shimerman's character is a tech geek who manufactures authentic props for costumes at sci-fi conventions. Donnely will be a little too eager to show Beckett how all his guns work.
In addition, Castle has also cast Christina Moore of That '70s Show and Hawthorne to play Stephanie Stern, the actress who played "Lt. Chloe" in this Firefly reminiscent episode. Moore's character, Stern, is said to have gone on to a successful movie career.
Source: TV Guide