'Castle': Seamus Dever Teases Wedding Troubles, 1940s Noir Episode and Pranking Nathan Fillion

Castle Seamus Dever Nathan Fillion - 1/9/12 - H 2012
Adam Larkey/ABC

Castle returns to ABC after taking the holidays off and front and center is Det. Kevin Ryan's upcoming nuptials to fiancee Jenny, but in true Castle fashion, something potentially life-changing forces the couple to rethink their future.

Actor Seamus Dever said as much when he chatted with The Hollywood Reporter about Monday's pivotal episode, centered on the murder of a lothario who has surprising ties to Jenny. "She becomes involved in the murder and we discover some information that might affect Ryan's decision to get married at all," Dever tells THR. That's not all: The couple will have another point of contention.

"Ryan and Jenny have decided to go on a cleanse to purge themselves and make themselves pure before they get married. It's sort of a spiritual thing," Dever says.

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The cleanse proves to be too much. "He's letting things boil and he's becoming anxious because he can't wat. His partner Esposito is taunting him with donuts," Dever says with a laugh. So much so that by episode's end, "Ryan decides to finally break his cleanse. There's a scene where everybody's eating Chinese food and Ryan loses it and starts devouring everything on the table," he adds.

Monday's episode is crucial to Ryan's journey to becoming a man -- and playing opposite his real-life wife Juliana helped. He recalled a moment that drew parallels to his actual wedding: "There's a moment right before I head into the chapel to get married; it's a funny thing that happens to a person. There's a mixture of excitement, of nervousness and fear all combined into one."

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As for the film noir episode, Ryan may not be the person viewers have grown to love for the past three and a half seasons.

There are "flashback scenes to the 1940s where Castle discovers a private detective's journal, written like a Raymond Chandler novel," Dever shares. "We're all in there, but we're all opposites of what our personalities are." Who does Dever play? "I get to be this gangster, tough-ass guy with an Irish accent."

For the episode, the cast shot at the Park Plaza Hotel in downtown Los Angeles, a place Dever worked at as a cater waiter. It was during production that he and fellow actor Jon Huertas pulled a fast one over co-star Nathan Fillion.

"They have a dirty basement that Nathan and Jon and I explored. Nathan went down there on his own but Jon and I snuck up behind him. Jon used his special forces skills and we scared the crap out of Nathan," Dever recounted with a laugh, not offering any more details. "It was awesome."

The big two-parter this season will see former The Chicago Code star Jennifer Beals joining as the guest CIA agent. "They spend a lot of money on those two-parters. I read the script and there's a lot of cool stuff that happens in stunt terms," Dever says. "There's a big cliffhanger at the end of episode 15."

Castle airs Mondays at 10 p.m. on ABC.