I loved this scene in "Nikki Heat" (3.11), an episode in which B-movie actress Natalie Rhodes shadows Beckett (just as Castle himself does--the series' whole premise) in order to prepare for her role as Nikki Heat in the film adaptation of Castle's novelized version of the real Beckett.
At first, she doesn't even know who Castle is (she hasn't read the book), but later she tries to seduce him--unsuccessfully--as part of her prep. Beckett finds Rhodes increasingly aggravating and is dismayed looking at herself in the actress/mirror, which leads to this delightful exchange.
Beckett: [Watching Natalie at the murder board] Do I really do that?
Castle: Yes, and it's adorable.
Beckett: If it's so adorable, why didn't you sleep with me? [Castle looks momentarily puzzled] Her me, not me me.
Castle: Oh, well a fictional character that I wrote, based on you, played by Natalie Rhodes? It's just way too... meta.
