Adam on "Castle" Season 4 (Spring 2012)

Long before Nathan Fillion was solving crimes with Stana Katic on Castle, he was soaring through space on the short-lived cult fave Firefly. Now, for the first time, Castle is reuniting Nathan with one of his former co-stars on the ABC crime series.

In an episode slated for spring, Adam Baldwin, who played Firefly's Jayne Cobb to Nathan's Capt. Malcolm 'Mal' Reynolds, will guest star as a detective named Ethan Slaughter. (How's that for a character name?)

Slaughter is described to me as a rough and tumble cop from the city's gang unit who lands a case that Castle is desperate to follow. Along the way, Slaughter shows Castle a dirtier side of police work that forces Castle to reconsider what he's willing to do to close a case. As the season nears its cliffhanging conclusion, expect this new alliance to place Castle in the line of fire like never before.

My sources tell me Nathan is over the moon about this fun nod to his sci-fi past. Nathan and Adam also co-starred together in Firefly's 2005 spin-off feature film, Serenity.

'Castle' boss on the next few 'challenging' episodes, plus scoop on Adam Baldwin's guest spot
by Sandra Gonzalez

Richard Castle’s hurting — and it’s not easy to watch. In fact, it’s downright heartbreaking.

After learning in last week’s episode that Beckett (Stana Katic) remembered everything about being shot (including, you’ll recall, Castle’s confession of love), Castle (Nathan Fillion) is looking for a way to deal with an influx of disappointment. His solution? Apparently flaunting a blonde in front of Beckett in tonight’s episode! But there’s so much more underneath his playboy guise, says executive producer and creator Andrew Marlowe.

“I think he feels like if he confronts her, it’s just gonna be a big ego blow, so it’s better for him to swallow it at this point, which gives us a little bit of drama in the next couple episodes,” he says. And with only four episodes (including tonight’s) left in the rest of the season, the drama will come fast and furious, stemming first from the fact that poor Beckett has no idea why Castle is giving her the cold shoulder. But her confusion and — in a way — her sense of loss from his apparent “drifting away” could prompt Beckett to make some bold actions. “She’s wondering if she needs to declare herself, and she’s feeling ready…but then he’s acting out. So his behavior is going to be a little challenging to her, and it puts us in an interesting place for the next couple episodes,” Marlowe says.

Interesting, but difficult, considering the Beckett/Castle relationship and banter is the bread and butter of the show’s appeal. “I gotta say, the show is really easy to write when the two of them are getting along, because that’s the DNA of the show — when they’re giving each other s–t and they’re locked in the banter. When they’re on the outs, it’s always been a challenge,” he says. But “since we know where we’re going, I don’t think that we have the anxiety that a lot of the fans do.”

Luckily, a guest star in next week’s episode is likely to diffuse a little of the fan tension: Fillion’s Firefly co-star Adam Baldwin, who guests as Castle’s new creative inspiration, Det. Slaughter. “Because of where he’s at with Beckett, [Castle] is feeling a little uninspired. So he sees this guy on the news, who’s a larger-than-life figure, and he thinks, ‘Well maybe I need a different muse. Maybe I need to go in a different direction for a bit,’” previews Marlowe. But might Castle be biting off more than he can chew? “He gets in a little over his head because Det. Slaughter is not the same kind of cop Beckett is. And so he finds himself in more challenging situations and wonders whether the grass is greener.”

The episode, adds Marlowe, allowed the writers to deal with the Castle-Beckett relationship in a unique way. “It’s all about the Castle-Beckett relationship, and Beckett ends up being very much a part of it,” he assures. “The two of them are going to be challenged and learn some new things, and it’s gonna be interesting as we get to the conclusion.” (That finale, by the way, is whimsically titled, “Always.”)

But before the finale, everyone has to survive the zombies. Yup, zombies. “Our second to last episode, despite the fact that we’re dealing with Castle and Becket being in a rocky place and dealing with those issues, we wanted to have a little fun. We’re tackling zombies since they’re so much in the zeitgeist. Zombie killers.” We assume he’s talking about costumes. Right? “Well, you got to tune in to see.”

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

"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," Baldwin tells THR of working with Fillion again.

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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.”

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.’ “

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.