Castle is concerned in this scene and for very good reason: On the episode "Still," Beckett steps on an explosive.
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Castle has a tough mission ahead of him here: Make Kate Beckett laugh and relax... even though she is standing on a bomb.
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Kate has taken one wrong step. As a result, she's now standing on an explosive.
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He's a Writer. She's a member of the Police. Together, they are the most entertaining couple on TV.
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Do not move, Kate Beckett. This is a very dangerous situation because the detective has stepped on an explosive.
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A member of the bomb squad takes stock of the situation in this still from "Still." Beckett has stepped on an explosive.
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DO NOT MOVE, KATE! The detective has stepped on a bomb in this scene from "Still."
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Castle tries to calm Beckett in this scene and for good reason: she has stepped on a bomb.
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Castle Season 5 Episode 22 Quotes

Did you see the hearts? It took me like six minutes.

Castle

You're powerless to resist me, you always have been.

Castle