Castle's Nathan Fillion and Guest Star Adam Baldwin Dish on Reuniting in Season 4, Episode 21: "Headhunters" (VIDEO) Tweet April 11, 2012 by Wetpaint Entertainment Staff Related: Castle, Guest Stars , Nathan Fillion , Season 4 Episode 21 Credit: Access Hollywood Take a stroll down Firefly memory lane as former co-stars Nathan Fillion and Adam Baldwin reunite after 10 years on the set of Castle! Original Air Date: Castle Season 4, Episode 21: "Headhunters" airs on Monday, April 16, 2012 at 10 p.m. ET on ABC.
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