'Castle': Adam Baldwin Teases 'Firefly' Nods, Reuniting With 'Uber-Nerd' Nathan Fillion

Ron Tom/ABC

"Castle"

Richard Castle, meet Ethan Slaughter.

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Adam Baldwin guest stars on Monday’s episode of Castle as the “rogue” and “colorful” New York City detective Slaughter, who finds himself babysitting – more or less – the novelist on the latest case.

The unlikely Castle-Slaughter team-up comes at a pivotal moment for the series, as Castle (Nathan Fillion) figures out his next move now that Beckett’s secret (that she remembers everything, including his “I love you”) is out. Creator Andrew Marlowe teased as much when he spoke to The Hollywood Reporter last month: “When some of the information is put on the table and they start making discoveries, then it's going to cause them both to do a little soul searching.”

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Baldwin’s appearance also marks an anticipated onscreen reunion for the old Firefly pals. (Fillion played Mal, while Baldwin portrayed Jayne on the one season-and-done Fox sci-fi series.) Baldwin, who wrapped up five seasons as John Casey on Chuck, told THR that there “will be a couple” of Easter eggs thrown into the episode for followers of the Firefly universe. “They won’t be too over-the-top, hitting you in the head by a two-by-four,” he admitted, “but I think people who have seen Firefly will recognize a couple of winks and nods, yes.”

One may have already been revealed in a preview featuring Castle giving up his leather brown coat for Slaughter as he weasels his way into the action. (Watch the scene below.)

Fillion first introduced the possibility of Baldwin boarding the show by way of a mysterious text.

“I had gotten a cryptic text message from Nathan saying, ‘Is it possible that perhaps you might join us if there was a character that was appropriate for you to play?,’ “ Baldwin recounted. “And I said, ‘Yea, I’d jump at the chance!’ So when the creators came up with the role, I said, ‘Great, send me in there.’ “

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When it came time for Baldwin and Fillion to get down to business, the two picked up right where they left off. “It’s just so much fun hanging out with Nathan and do scenes with him on the set and have him show me how an iPad works,” Baldwin joked. “He’s like an uber-nerd gadget guy and I’m a little bit of a techie but not as much so it’s helpful to have him nurture me along there.”

Baldwin reminisced about on-set moments he and Fillion shared during the Castle shoot. “We had certainly bred a lot of familiarity and love for each other so it was like an old pair of comfortable boots that you could put back on and kick it around a bit,” he said. “He and I have been friends for 10 years. It’s just so much fun to look over at his face and see him smirking at me. I say, ‘What are you thinking?’ And he goes, ‘Remember that time …’ ”

“I felt very at home,” Baldwin added, after naming crew members from past projects like The X-Files, Chuck, Firefly and Serenity who moved over to the ABC procedural.

Like his Firefly character, Baldwin’s Slaughter is rougher than what Castle may be used to with usual partner Beckett.

“He takes Castle into the line of fire to show him the ropes and I think he kind of likes the writer fella that Castle is,” he teased. “He lives on the edge, carries a big weapon – I don’t know if that’s overcompensating for something but I’m not even going to speculate on that one.”

“There’s so much tongue-in-cheek and wonderful humor that goes on interspersed amongst the serious stuff that a crime show and a procedural can have,” Baldwin added.

But how similar is Slaughter to Chuck’s John Casey or Firefly’s Jayne? “Yea, [he’s] a confluence between the two characters, sure, but he doesn’t have John Casey and Jayne’s accent -- more of an urban street-type thing,” as Baldwin puts it.

For him, there’s one scene that may give viewers reason to tune in.

“There’s one interrogation scene in particular that I think people will get a kick out of. Slaughter’s tactics are not exactly orthodox, or not necessarily what Castle would approve of, yet they get results,” Baldwin says with a laugh.

Watch a scene between Fillion and Baldwin below:

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