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 (10 p.m. Mondays) 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 its star as different. "I feel like it's a new series entirely," says Katic. "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," she says. "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 a recent 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.

Marlowe quickly learned that his new discovery brought more than charisma to the table.

"She's great in the day-to-day needs of the show," he says. "She's a great, strong cop who can walk in a room and project authority. She also owns her femininity. And when we need her to access those darker emotions, she is really incredible."

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?

"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.

"I wonder if that's going to be enough for the Beckett character. She's starting to think of him as a future husband, a possible father. She's complex and loving and twisted and all of the wonderful contradictions I think almost every woman is. I think a woman like that is going to demand a partner and a lover with depth as well."

The more you learn about Katic's background, the more you understand why substance might be an issue.

She was born to a Croatian mother and a Serbian father in a mill town near Toronto. When she was very young, they moved to the Chicago area, where her parents started a furniture store and had five more children.

"My parents were European immigrants," she says proudly. "They came to the States with $1,500, two suitcases, and me, and they managed to build a business, a family, and a future for their family. They didn't have any of the resources of people who have lived here for two or three generations."

One legacy of her childhood: If you want someone to shop for a dinette set with, Stana's your gal.

"I'd be good at figuring out which pieces would last for a while," she says and laughs. "I'd help get it in the house, up the stairs, and around the corners. I've helped a few friends move. It's shocking how that weird geometry stays with me."

It's not too hard to tell she was an oldest child, either.

"I'll see Stana running up and down the lot with some kid and I'll say, 'Who is that?' just assuming it's one of her relatives," says Tamala Jones, medical examiner Lanie on "Castle." "And she'll say, 'Oh, this? This is so-and-so's daughter.' All the producers bring their kids to the set and they're all in love with this girl. They're all, 'Staaana!' "

'castle'

When: 10 p.m. Mondays
Where: ABC