Castle is definitely planning a Comic Con themed episode this season and now new news has surfaced that Nathan Fillion's Firfly past will play a part in this upcoming season 5 installment. Let's just say that whoever they cast will have beautifully tight pants to fill ;)
Castle will also be taking inspiration from star Nathan Fillion's Firefly past. The episode features a past-his-prime sci-fi actor who, despite once playing the beloved "Capt. Max Richards," is now a promiscuous alcoholic who comes under suspicion when a fan is murdered at a convention.
Source: TV Guide