Castle Castle

Monday's Castle is taking its characters (and viewers) back in time.

Exclusive: Castle boss and Stana Katic dish on Castle's first muse

As Castle (Nathan Fillion

) and Beckett (Stana Katic) investigate the murder of a treasure hunter who was searching for the elusive "Blue Butterfly" necklace before his death, the pair stumbles onto the victim's guidebook: a diary from a 1940s private investigator named Joe Flynn.  It's not long before Castle brings the old text to life by imagining himself as the P.I. who is trying to save a beautiful young woman named Vera (Beckett, naturally) from the notorious gangster Tom Dempsey (guest star Mark Pellegrino). Since Castle and Beckett are forced to find clues from the past to help them solve their current murder, viewers get to see Joe and Vera's twisty, tragic love story play out. Still need convincing? Here are five reasons we think the episode is worth a look.1. Lanie sings! In Castle's imagination, medical examiner Lanie Parish (Tamala Jones) becomes a Billie Holiday-like singer at Dempsey's club. "That was fun!" Jones tells "I have never sang before outside of the shower. When I was asked to do that, I was a little nervous. But I shocked myself. I'll let you guys hear for yourself, but I think I did a good job." We agree.

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2. The show nails the look and sound of the era.
The writers, set-dressers and costumers outdid themselves here, which helps all the characters come alive in the past. From Vera's long gowns and furs to Joe's trench coat and Raymond Chandler dialogue, the show does the genre proud. There may have been one downside, though. "I got to wear a lot of crap in my hair," says Seamus Dever, who plays Detective Ryan. "I had to wash it five times every night to get it out. But it was nice."

3. Speaking of Ryan and Esposito (Jon Huertas)... Castle imagines his other partners in crime-solving as two of Dempsey's thugs. "Esposito and Ryan get to rough Castle up a little bit," Dever teases. Jones says playing characters so different from what the actors are used to is part of the fun of the episode. "If you morphed the Castle characters into the 1947 characters and they're basically playing the same simultaneously, it kind of makes it boring," she says. "What you see, you're going to love."

4. There's a kiss! Actually, there are two kisses. Will it be enough to satisfy the Castle-Beckett 'shippers? "The '40s episode is probably the fulfillment of what most any fan would like to see happen with the Castle and Beckett relationship," Katic tells us. "Fans will be seeing the ultimate expression of love for those two characters."

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5. You can watch (some of) the episode now!
Why wait until tonight when you get a taste of the fun now? Check out the video below!

Castle airs Mondays at 10/9c on ABC.