Adam Baldwin and Nathan Fillion Adam Baldwin and Nathan Fillion

Just how easy was it for former Firefly co-stars Adam Baldwin and Nathan Fillion to find their old rhythm while shooting Monday's episode of Castle?

"It was like putting on an old pair of comfortable boots," Baldwin tells with a laugh. "All those old feelings from Firefly and 10 years of working together come rushing back with just a look, wink and a nod."

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Baldwin plays Detective Slaughter, a rough-and-tumble cop from the gang unit who agrees to take Castle (Fillion) under his wing for a new case. "He's a colorful character," Baldwin says of Slaughter. "He's kind of a rogue cop. He takes a few liberties with procedure, which Castle doesn't necessarily approve of, but he gets results. And he has nice clothes and a big gun and a fast car. He's kind of a cliché, but it's a fun opportunity to chew some furniture."

Slaughter is a welcome distraction for Castle, whose relationship with Beckett (Stana Katic) has been strained ever since he learned that she remembered everything after her shooting in the season finale — including that Castle said, "I love you." "Castle's looking to recreate the magic he had with Beckett and he is looking for inspiration, so he decides to follow another cop around," creator and executive producer Andrew W. Marlowe says. "And it's kind of: Is the grass is always greener? What kind of lessons does Castle learn when he's out there running and gunning with Detective Slaughter?"

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One thing that's instantly clear: Castle is in over his head. "Let's just say that all of the characters have a very strong point of view on who Detective Slaughter is and what Castle is getting himself into," Marlowe says. "And when Castle tries to impress Slaughter with all of his police connections, he learns pretty quickly that Esposito [Jon Huertas] and Ryan [Seamus Dever] don't work for him."

Baldwin says Castle's political connections play a major part in why Slaughter indulges Castle. But the pair does become closer, and eventually, Castle will open up about his troubled relationship with Beckett. "Does Slaughter have some advice in the lovelorn department? You bet," Baldwin says, with a huge laugh. "He's an outspoken guy. He definitely mentions some things in a not-very-diplomatic way."

So, will Slaughter's advice inspire Castle to talk things through with Beckett? "Men in general usually don't confront their feelings," Marlowe says. "He's certainly not the type of person to go up to Beckett and say, 'Hey, you hurt me!' because that would be exposing himself. He's expressing his anger with Beckett, and whether he knows it or not, he's trying to punish her without really putting his cards on the table."

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And for all the Browncoats, Baldwin says there are a few subtle winks to his and Fillion's Firefly past. "We certainly played around in several takes, and if they made it into the final cut, that's the producers' choice," he says. "We certainly tried." Adds Marlowe: "We were aware that if we did anything too overt it would take people out of the story. I think we're looking to take the characters beyond that."

Castle airs Monday at 10/9c on ABC.