Nathan Fillion and Stana Katic Nathan Fillion and Stana Katic

"So, it wasn't a dream!" Richard Castle says early on during Monday's Season 5 premiere of ABC's Castle.

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Castle (Nathan Fillion

) is, of course, expressing his excitement about having finally hooked up with his muse, NYPD Detective Kate Beckett (Stana Katic) after four long seasons of will-they-won't-they drama. Had it all been a dream, it would have been a nightmare scenario for fans — and for the show's creator, Andrew W. Marlowe."We had kept them apart as much as we could while still being credible," Marlowe tells "Keeping them apart would have felt artificial and would have damaged the show."But before anyone even utters the words "Moonlighting curse," Marlowe wants to make one thing clear: He's confident the hookup and its implications will change very little about the way Castle and Beckett interact. "These two people are strong-willed individuals," he says. "They are not going to change who they are because they're in a relationship. All the stuff that got on each other's nerves before will certainly get on their nerves now."

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But things are different. Instead of dancing around their feelings for one another, the crime-solving duo will actually have to express those feelings to one another. And that may not always go as they might've hoped. "We expect to have an interesting honeymoon period, where the two of them are dealing with what this new thing is and how they feel about it," Marlowe says. "Is it real? Are they in a relationship? Are they in an exclusive relationship?"Beckett always had this fear of being another notch on Castle's womanizing belt," Marlowe continues. "And Castle has a fear of getting into a relationship because he's done it twice and it hasn't worked out. I think both these characters still have some internal issues that they're dealing with. When we get deeper into the season and they're having a conversation about the future of the relationship, I think that's a terrifying question for both of them."But there is also a terrifying issue at hand in the here and now. Last season, Cole Maddox (guest star Tahmoh Penikett), the man who shot Beckett in the Season 3 finale, vowed to "put Kate Beckett in the ground for good." And he wasn't lying. "Beckett is still under threat," Marlowe says. "When [they] wake up the next morning ... in that blissful bubble that they may be in, life has a way of coming in and bursting that bubble. Real life intrudes really quickly, and that is part of Beckett's journey."

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Even though Beckett resigned last season, she'll quickly get back on the case once she learns Maddox is still after her. The resulting investigation leads Beckett to Sen. William H. Bracken (guest star Jack Coleman), who Marlowe says has some "compelling information" about Beckett's mother's murder. But now that she's closer to the truth than ever, Beckett's actions may surprise you. "We follow a trail and discover that [Bracken] is somebody that has some insight into what's going on," Marlowe says. "It's going to spin our characters in a different emotional direction. Once Beckett has that knowledge, what's she going to do with it and who is she going to become?"With the prospect of finally putting her mother's murder behind her, will Beckett finally be fully able to commit to Castle? And can Castle turn away from his millionaire playboy lifestyle? These are the questions that Marlowe promises will keep the tension between Castle and Beckett as thick as ever.

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"People fall in and out of bed with each other all the time," Marlowe says. "But what we're dealing with is a much more emotional trajectory for these two, and we're nowhere near resolving it. There are a lot of hurdles to overcome. Some of them will be really funny and some of them will be serious."There's still a fun journey ahead of us," he continues. "None of us feel like we've reached any kind of finish line. We've certainly reached a milepost, but to us this is the beginning of something, not the end of something."Castle premieres Monday at 10/9c on ABC.