This season of “Castle” strikes one of its stars as different. “I feel like it's a new series entirely,” says Stana Katic, who plays NYPD homicide detective Kate Beckett.

“Castle” 10 tonight, ABC

Blame it on love. After some exquisitely protracted flirtation, roguish mystery writer Richard Castle (Nathan Fillion) and keen NYPD homicide detective Kate Beckett (Stana Katic) are finally exchanging pillow banter in the fifth season of “Castle.”

And ratings for the ABC series have never been better (an average of 14.1 million viewers). Or maybe it’s not the sex. Because the audience for this intoxicating and unique comedy-romance-crime procedural hybrid has grown every year it’s been on the air.

In any event, this season strikes Katic as different. “The tone is lighter now that the two characters are together. It’s a treat to go to work.”

Two side effects of the passion plot line: Fans of the show, determined to come up with a cute pet name for the couple, have settled on the unfortunate “Caskett.”

And the new intimacy means that in many scenes this year, Katic, 34, is wearing less clothing. “It’s a balancing act to make it sensual and keep it modest enough for network television. The other element is I’m protective of the character. She’s the romantic lead, but I don’t want her objectified, especially since our audience is so heavily weighted with females. I definitely want her to be respected.”

This being “Castle, even the bedroom scenes can be a little offbeat. In last week’s episode, a hot-and-bothered Castle watched as Beckett did a slowly revealing Gypsy Rose Lee dance in a “Star Trek”-like sci-fi costume (it’s a long story). When she finally emerged, she was wearing a hideous alien mask, putting a real and immediate damper on the mood.

Farcical flourishes like that validate “Castle” creator Andrew Marlowe’s original gamble on casting Katic. Fillion was the tent pole of the show, because of his strong cult popularity from roles like Captain Mal on “Firefly.” The lesser-known Katic was chosen after an exhaustive talent search because she displayed evident on-camera chemistry with Fillion.

The unexpected bonus: Katic’s comic chops, which give the dialogue its distinctive tom-tom rhythm.

“Nathan is a fantastic comic talent,” says Marlowe. “So a lot of time, Stana has to play the straight man. But each has their own area code for humor. He has the one-liners, the bon mots. She has the drier humor, the cutting observations.”

OK, no more kidding around, Marlowe. Is there a future for, er, Caskett? Or is this just a November sweeps romance?

“As long as it’s interesting,” is all he’ll say.

Katic is even more dour about the couple’s prospects. And it’s because — get this — she thinks Castle isn’t man enough! “The Castle character has always been something of a cad — very funny, very charming, but as soon as the question of more depth comes up, he bounces,” Katic says.