A scene from “He's Dead, She's Dead”

Kate Beckett: [after she and Castle confront the murderer] Did you figure out that Emilio was cheating, or did he tell you?

Paula Casillas: He told me that he had met someone. That she was young and beautiful, and that I was just a boring old lady. I took some selenium from the factory, and every morning I put a little bit in his coffee.

Marina Casillas: My god. Mom?

Paula Casillas: When you wanted to go see Vivien, I went along to humor you, but she knew things that she shouldn't have known.

Kate Beckett: And then when she told you that Emilio wanted to tell you something important...

Rick Castle: ...and she said she was going to spend the evening communing with his spirit...

Paula Casillas: The cheating son of a bitch was gonna tell on me. After everything that he had done, I couldn't let that happen! I had to kill her to stop him!