Monday's episode of Castle offers a scene straight out of the title character's fantasies: Castle is alone in a room with two Becketts!Nathan FIllion ) and Beckett (Stana Katic) are being shadowed by Hollywood starlet Natalie Rhodes (guest star Laura Prepon), who has been chosen to play Nikki Heat, the character Castle based on Beckett, in the movie adaptation of his novel. But Castle's imagination doesn't run wild right away. In fact, he thinks Natalie, best known as a horror movie scream queen, is completely wrong to play his beloved character."At first, he's like, 'I can't believe this actress is playing Nikki Heat,'" Prepon tells TVGuide.com. "But my character starts to grow on everybody and prove herself. At first he's not into it, but he gets into it and is confused because he's secretly in love with Beckett."Beckett, on the other hand, is thrilled to have Natalie follow her around — at least at first. "She shows up and wants to follow Beckett to get an understanding of what the character's about," creator Andrew W. Marlowe says. "Beckett is very excited because she's a fan, but then over the course of the episode, it feels a little bit to her like Single White Female, like this woman is taking over her personality."duo's first kiss is drawing nigh."What's cool about playing someone who's coming into the situation totally objectively is that she can totally see, like, 'Oh my God, these two are totally into each other but are not admitting it,'" Prepon says. "It's fun to be the character that shakes things up and gets them both to realize [they] should do something about it. [She's] that objective observer who can lay it out for them and stir up the pot a little bit."Castle returns Monday at 10/9c on ABC.