Nathan Fillion and Dana Delany just can't stay away from each other.
On the short-lived 2001-02 Fox series "Pasadena," they played Glenn Collins and Catherine McAllister, who had an illicit affair.
When Delany landed on ABC's "Desperate Housewives" in 2007, playing Katherine Mayfair, there was Fillion as her younger husband, troubled gynecologist Dr. Adam Mayfair.
Now it's 2010, and Delany is still "Desperate," but now Fillion is the star of his own hit ABC drama, "Castle," which premiered in 2009 and airs at 10 p.m. Mondays.
He plays mystery novelist Richard Castle, a charming rogue who, through his friendship with the mayor, has insinuated himself into the crime-fighting efforts -- and life -- of ambitious NYPD Detective Kate Beckett (Stana Katic).
On Monday, Delany and Fillion give it another go in a two-part episode of "Castle" (Part Two airs March 29), but this time, if romance is involved at all, it's not theirs.
Delany guest stars as Jordan Shaw, an FBI agent who, along with her partner (Leonard Roberts), teams up with Castle and Beckett to track an elusive killer.
"I'm sort of Beckett, older," says Delany, taking a break between shots on the show's New York police precinct sets (actually, on this day, in a very rainy Hollywood). "I come in, I'm successful. I'm good at what I do, and yet I'm also married and have a kid.
"She's been thinking she can't have it all, and she shouldn't get romantically involved with Castle. I'm an example of somebody who's been able to juggle that. Not that it's perfect, but I am able to do it."
In an earlier scene, Shaw and Castle were batting ideas in the case back and forth before coming to a sudden realization and then charging off to follow up on the lead -- leaving Beckett looking (and probably meant to be feeling) like a third wheel.
"It's strange, for Beckett," Katic says, "first of all, to have someone else take over a case, because she's always the tip of the spear in all the cases. As we move on in the story line, we discover that she's the focus of a serial murderer's interest.
"Beckett has always been the one that protects the vulnerable, and suddenly she's the vulnerable one."
And, of course, Shaw's muscling in on Beckett's territory -- namely her odd relationship with Castle. As Fillion describes it, the two don't engage in normal courtship or dating; they get worked up instead over solving crimes together.
"That's their version of courtship," he says. "That's their version of foreplay. That's their version of romance.
"As you can see in the scene just now, Jordan Shaw starts seeing the value of Castle. The two of them start connecting on this level of, 'We're solving crime together.' There's a chemistry there. There's an electricity there.
"Beckett is, of course, as you would expect, 'What the . . . how . . . you don't . . . you and I are supposed to . . . that's not.'
"I like it. It's fun."
Delany played an FBI agent in an unsold pilot at the turn of the century -- co-starring, coincidentally, Julie Benz, her "Desperate" character's new love interest -- and she had to revive the skills she learned then for "Castle," more or less.
"We did a SWAT team scene," she recalls, "where I had on my high heels and a flak jacket on, and I'm running with my gun.
"One of the camera guys said, 'I really hope that if the FBI's chasing me, that it's you, because I think I could get away.' I said, 'I know.' "