By Ken Tucker
Updated January 03, 2011 at 12:00 PM EST

Castle had a clever little outing this week titled “Nikki Heat,” with guest star Laura Prepon playing an actress researching her Nikki role in Heat Wave, a movie being made from the novel of the same name by Nathan Fillion’s Rick Castle.

Prepon’s Natalie Rhodes arrived in the New York police station much as Castle himself did in the series premiere — she was there to shadow Stana Katic’s Kate Beckett for research. The humorous twist was that Castle had no respect for Rhodes as an actress (she was famous for B-movies with title like Mutilation High and Hell’s Crawl Space), while Rhodes had no respect for Castle since she hadn’t read his book, only the script someone else had written.

This was cute with a side-dish of media criticism. The episode had fun with the notion that, on a feature film project, the author of the source material is routinely lower on the cinematic totem pole than the crafts services company, and few people on TV play abashed humiliation better than Fillion. (“Hello? I’m your creator!”)

In essence, Prepon was playing Nikki based on Beckett as played by Katic. “Way too meta,” complained Castle feebly. Not at all. By the time Natalie had donned a Beckett-style wig and was imitating the cop’s posture and vocal inflections, she’d very nearly lived up to her promise to “do you better than you.”

I went to the movies over the weekend and caught a mirthless trailer for No Strings Attached, a forthcoming rom-com starring Ashton Kutcher, and wondered all over again why That ’70s Show, a perfectly decent sitcom but no classic, should have nurtured careers that include Kutcher, Mila Kunis, Topher Grace, and serial celeb-dater Wilmer Valderrama, while Prepon hasn’t really taken off in projects of great prominence. (You’ll excuse me if I don’t count a stint on October Road as being “of great prominence.”) I thought the same thing while watching Castle: Prepon came across as smart and appealing; the idea of her having a lower profile than Kunis is a head-scratcher. (Not that I’d wish Black Swan on Prepon, but you know what I mean.)

As usual with Castle, the crime under investigation wasn’t as interesting as the characters’ interaction. I used to foolishly complain about this; now I recognize that crime is not what Castle is about — it’s the personalities that either hold your interest or don’t. This week, those personalities shone brightly: The invasion of an outside agitator as played by Prepon brought out the best — actually the least meta, least ironic, and therefore most enjoyable — aspects of Castle and Beckett.

Did you watch? Do you agree, or disagree?

Twitter: @kentucker