Richard Castle should be pleased: A big movie star (played by 'That '70s Show's' Laura Prepon) will be his heroine in "Heatwave" -- the film adaptation of his book series featuring the character Nikki Heat.

But it's NYPD Detective Kate Beckett -- Castle's muse for Nikki -- who inspires Natalie Rhodes. "You are exactly what I expected Nikki Heat to look like," she gushes to Beckett on 'Castle' (Mon., 10PM ET on ABC). "I'm very method, so it's all about getting inside of you, which starts with observing and noting."

"Observe and note away -- I'm already used to it," she says dryly, in Castle's direction.

As for the author/screenwriter, Rhodes isn't starstruck in the least. "I wrote 'Heatwave,'" Castle tells her. She replies, "Oh, I thought your name sounded familiar..."