Castle Season 4, Episode 21: "Headhunters" doesn't air until April 16, but it's never too early to get excited about another hour-long installment of our favorite show — especially when it features a Firefly reunion!

Check out these behind-the-scenes pictures of Castle's Nathan Fillion with former Firefly co-star Adam Baldwin, who guest stars as rough and tumble gang unit detective, Ethan Slaughter.

Credit: Randy Holmes/ABC Television Group © 2011 Disney Photo: Nathan Fillion and Adam Baldwin Shoot a Scene in Castle Season 4, Episode 21: "Headhunters"
Credit: Ron Tom /ABC Television Group © 2011 Disney Photo: Nathan Fillion and Adam Baldwin Share a Laugh on the Set of Castle Season 4, Episode 21: "Headhunters"
Credit: Ron Tom /ABC Television Group © 2011 Disney Photo: Nathan Fillion and Adam Baldwin Reunite on the Set of Castle Season 4, Episode 21: "Headhunters"

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