Lisa Edelstein, Stana Katic, Jocko Sims

Castle's Season 5 finale pushed Castle and Beckett's relationship to the brink. After Beckett (Stana Katic) received an offer to work in Washington D.C., Castle (Nathan Fillion) became furious that Beckett kept the interview process a secret from him.

But just when it seemed the pair was moving in different directions, Castle surprised Beckett — and viewers — by proposing. "It just became clear that it was the right time for this couple," creator and executive producer Andrew W. Marlowe tells TVGuide.com about the game-changing decision. "They were coming to a point in their lives where they were asking questions. Things come up that challenge you, and you start looking at definitive choices that you're going to make that are going to send you down one road or another."

Fall Preview: Get scoop on more than 40 of your favorite returning shows

But does Beckett's road lead to D.C.? Marlowe promises viewers that Beckett's responses to both the proposal and the job offer will become clear very quickly on Monday's premiere (10/9c, ABC). But the answers to those questions is just the beginning of this season's drama.

"Whether she takes this job in D.C. and whether she says yes or no to the proposal, those are significant choices in her life," Marlowe says. "Any choice she makes is going to fundamentally alter her relationship with Castle, one way or another. There's no version of this that's easy and that they live happily ever after — even the good stuff. ... There are four possible outcomes and each of them have their own particular consequences. We're dealing with the consequences of those decisions all season."

While some fans questioned the purity of Castle's intentions — did he propose just to try to keep Beckett from taking the job and leaving him behind? — Marlowe doesn't see it quite that way. "I think that he asked his question out of emotion and passion and the circumstances of the moment. You can't think everything through," he says. "[Castle] was challenged to think about what he really wanted in life. I think his preferred version is that he has this woman in his life."

Exclusive Castle first look: Are Castle and Beckett dressed for a wedding?

Does that mean Castle, who's walked down the aisle twice before, is emotionally mature enough to make marriage work this time around? "I think he very much likes to think that was the case, but he also is who he is," Marlowe says with a laugh.

For Beckett's part, she will have to decide if "who Castle is" is the person with whom she wants to spend her life. Helping Beckett examine that question is Rachel McCord (guest star Lisa Edelstein), a federal investigator who becomes a mentor of sorts to Beckett during a three-episode arc. And it's safe to say McCord doesn't initially think very highly of Castle. "People come with a lot of baggage, and she's starting to realize what Beckett's baggage is," Edelstein says of her character. "McCord views Castle as a threat to Beckett. He is hampering her potential."

Although McCord initially comes around on our favorite mystery writer, she still advises Beckett about the dangers of mixing her personal life with her career. "I think she warms to Castle," Edelstein says. "She gets who he is, but she also knows what her job is. McCord has made mistakes of her own, and that's why she feels qualified to help Beckett avoid doing the same thing. She has definitely chosen her career as a priority to the degree that she would never have a relationship like Beckett has with Castle."

Castle books House alum Lisa Edelstein for Season 6

But will Beckett take McCord's advice to heart? "It's not really about whether Beckett and McCord see eye-to-eye about this stuff," Edelstein says. "It's part of their journey together. My story line is about Beckett's journey to having to make a very serious decision about her life."

And whatever that decision is, Marlowe suggests that viewers shouldn't lose faith in the show's core couple. "We played around with it and thought it through," he says. "We asked, 'What's honest and true? How can we surprise the audience and keep them on their toes?' ... We owe the audience a response so that we're not stringing anybody along."

So, should we expect to hear wedding bells this season? "We're telling a great love story," he says. "I think that if we don't deliver on some aspect of a satisfying conclusion to that at the end of the day, it'll frustrate people. But I don't think that we're necessarily near that particular conclusion."

Castle premieres Monday at 10/9c on ABC.

Check out our fall TV crushes!