Rick Castle's (Nathan Fillion) books are best-sellers and he has a long list of fans, including criminals. It does not happen often but at times, some fans do make him land in a dangerous situation. In "Castle" Season 6 Episode 4, "Number One Fan," Castle has to don the hat of the hostage negotiator because of his fan. Emma (Alicia Lagano), a fan of Castle, is holding hostages at gunpoint and she would only negotiate with Castle, the writer.

The instable lady does not trust the cops and wants Castle's help to prove her innocence in a muder case. She wants Castle to come inside the dental office, where she has taken hostages, to have a one-to-one talk. "I need to speak face to face, do you hear me," Emma could be heard saying on the phone in the sneak peek.

SNEAK PEEK: "Castle" Season 6 Episode 4: "Number One Fan"

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In the new sneak peek, Kate Beckett (Stana Katic) asks Castle what was he thinking when he struck a deal -- release of two hostages and in return, he will have a face to face talk with her. Castle feels that since Emma is his fans, she won't hurt her. He is confident that he knows his fans but this is fan is different

The first sneak peek of "Number One Fan" provided a glimpse of the interaction that takes place between Castle and Emma. Emma was disturbed and unstable in that sneak peek. It looked like she would pull the trigger any moment. Castle has to prove her innocence to ensure the safety of the hostages and himself, as well.

"CASTLE" - "Number One Fan" - A desperate young murder suspect, Emma Briggs, (guest star Alicia Lagano, "The Client List") takes hostages at gunpoint then mysteriously insists that she will only negotiate with one person: Richard Castle," according to the official synopsis of "Castle" Season 6 Episode 4 released by ABC.