Not too long ago, Nathan Fillion played Dana Delany's husband on "Desperate Housewives." Now, Dana is joining Nathan for a two-part story on "Castle." Not as his wife, but as federal agent Jordan Shaw, who teams up with Castle and Beckett (Stana Katic) to hunt down a cunning serial killer.

"He called me up when I was in the loony bin on 'Housewives' and said, 'You know, there's a part coming up and you would be great. I'd love you to do it.' And I said to him, 'Nathan, I think I already have a job,' but ABC very nicely worked it out," Dana tells ET.

And Jordan Shaw isn't just any FBI agent. Nathan compares her to Clarice Starling ('The Silence of the Lambs'), who is a superstar at catching serial killers.

"He's incredibly impressed by her and it does not go unnoticed by Beckett, who eventually she sees him and goes, 'Hey. You're supposed to be on my team.' He's like, 'We're all on the same team.' He's just infatuated," Nathan says.

As a child, Dana wanted to be an FBI agent and had even toured Quantico, but that was before she learned that there were no female agents.

"J. Edgar Hoover didn't want them," she explains. "In the '20s, there were three or four female agents, and he got rid of them. So it wasn't until he left that they came in. I think you might make a little bit more money as an actress. But I'm not sure. Plus, it's safer," she says about rethinking her career.

Dana admits she is not above adding a little romantic tension between Castle and Shaw, especially if that means a return engagement.

"I would love to have a recurring role as an FBI agent," she says. "That would be a blast to be able to come in and carry a gun."

"Castle" airs tonight at 10 p.m. on ABC.