Nathan Fillion, Stana Katic and Jennifer Beals

Jennifer Beals wants to make one thing very clear about her guest-starring role on Castle:  Her character isn't looking to rekindle her past relationship with Castle (Nathan Fillion).

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On Monday's episode (10/9c, ABC) — the first of a two-parter — Beals plays CIA Agent Sophia Turner, who served as the inspiration for the female agent in Castle's series of Derrick Storm novels. And while Sophia's reappearance may push the buttons of Castle's latest muse, Detective Kate Beckett (Stana Katic), Beals insists her character's motives are pure.

"She's not trying to get [Castle] back," Beals tells TVGuide.com. "One of the things I said to [creator Andrew W. Marlowe] is that I'm not interested in playing a scenario where two women are fighting over the same man.  Sophia can introduce lots of complications and lots of tension, but I wasn't interested in a catfight."

Be that as it may, Sophia's involvement in Castle and Beckett's investigation will force that duo to evaluate the current status of their relationship. "Beckett has to wrestle with where she thought she stood in Castle's life and where she might really stand in Castle's life," Marlowe says. "I think it serves as a bit of a touchstone for them. It certainly brings up stuff in both of our characters about how they're handling the relationship and getting a sense of what they want, what they're afraid of, and what they hope might happen."

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Adds Beals: "Whatever feelings it brings up for Castle, that's his thing," Beals says. "But Sophia's trying to solve a case. She is trying to prevent the United States from going into World War III. She's not like, 'Can I rekindle my romance at this very moment?' There are bigger fish to fry."

Indeed, Sophia re-enters the picture because Castle and Beckett stumble upon a murder that seems to have been perpetrated by a rogue CIA agent who just so happens to be at the center of Sophia's much larger international investigation. Regardless of whatever feelings may or may not still exist between Castle and Sophia, Beals notes that the pair still make a great team.

"When people have chemistry, they have chemistry," she says. "I think it's very rare that it completely changes. So they do sort of pick up where they left off. They are still able to finish one another's sentences."

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Beals, who last season appeared as police superintendent Teresa Colvin on Fox's The Chicago Code, admits she was a bit hesitant about taking on another female law enforcer so soon. But she is able to appreciate the characters' differences.

"I have to say that as soon as I got the gun on and had to lead the troops, I had to fight like mad not to do a Chicago accent," Beals says with a laugh. "Teresa is definitely a scrapper and she's good with strategy, but Sophia is in a whole other class.  If there was a fight, my money would totally be on Teresa, but you cannot put anything past Sophia. She's able to keep a lot of different puzzles in her head at the same time. "

Does that imply that her character might have some secrets that will be uncovered? "She's CIA!" Beals says. "She'll do whatever she needs to do. She doesn't have any problem stirring the pot in order to get things done."

Castle airs Mondays at 10/9c on ABC.