Nathan Fillion and Stana Katic

As the saying goes, you can't unring a bell. So, can you unspeak the words "I love you"?

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That's the question looming over Castle, as the Season 4 premiere (Monday, 10/c, ABC) focuses heavily on how Detective Kate Beckett (Stana Katic) will react to a profession of love from her mystery-writer/partner, Richard Castle (Nathan Fillion), in last season's finale. Of course, Castle's "I love you" didn't come over a candlelight dinner, but in the aftermath of Beckett being gunned down by a sniper. So, there are plenty of easy outs, right?

"My goals were to honor everything that we had done in the season finale and not sweep anything under the carpet," creator and executive producer Andrew W. Marlowe tells TVGuide.com. "My intent was to confront all the issues that we brought up and deal with them in a way that ... was going to be both satisfying to fans and also a little bit frustrating for the ones who want Castle and Beckett to get back right away. We've taken their relationship to the next level of complexity. ... [There's] a lot more meaning in the Beckett and Castle relationship."

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Adds executive producer Laurie Zaks: "Once the genie is let out of the bottle, it can't be put back in. Even if it's not acknowledged verbally, psychologically there's going to be a difference."

It's in those psychological and emotional corners of the story that things get a bit tricky. For example: Will Castle even be able to see past his own complicity in Beckett's shooting — and by extension, the sacrificial death of former Capt. Roy Montgomery (Ruben Santiago-Hudson) — to move forward in a relationship with her?

"It was a very sobering event for Castle. Certainly he carries a measure of guilt," Fillion says. "Here's a guy who's not very keen to grow up anytime soon. But he's really been spear-heading this investigation. He's been kicking this hornet's nest around, thinking that solving it will get him closer to Beckett. But now he's losing friends, literally. It's going to force him to grow up a little."

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Also standing in the way of Castle and Beckett's possible love connection is a decrease in face time, now that new Victoria "Iron" Gates (Penny Johnson Jerald) has assumed command of the 12th Precinct and promptly kicked Castle out.

"He's not used to women being around him that aren't disarmed by him immediately," Fillion says. "Women love him, and when they don't, he takes it really hard. ... She won't give him half a chance. Even when he deserves credit, she will not give it to him. That really bothers him, and it's going to be a continuing battle."

But the true obstacle preventing a Castle-Beckett hookup is secrecy, as both Beckett and Castle will be forced to hide significant information from each other at the beginning of the season.

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"Everybody's got secrets this year," Katic says. "The first episode kind of explains how this next season will be directed. It explains in a really subtle way how the two characters feel about each other and what obstacles they might have to tackle in order to finally get to each other. I think it's really sweet."

Sweet, if not immediately satisfying to the show's ravenous fans. Katic can only promise that Beckett, who will also share some her feelings with a police-mandated shrink (Star Trek: The Next Generation's Michael Dorn) this season, is acting in the interest of her long-term happiness.

"She's making an adult decision not to dive into something because she doesn't want to screw it up," Katic says. " She has real, adult reasons for making the choices she makes. ... She cares for him so much. She's in love with him. She's dated other guys, but he's the only guy she's been in love with."

Castle premieres Monday at 10/9c on ABC. Watch a clip from the premiere: