'Chicago Fire': Jesse Spencer and Kara Killmer Dish on Casey and Brett's Potential Romance (Exclusive)

'They're enjoying each other's company,' Spencer tells ET of the brewing relationship.

Is Chicago Fire exploring a romance between Casey and Brett?

Paramedic Sylvie Brett (Kara Killmer) officially returns to Station 51 in Wednesday's episode, a bittersweet homecoming for the young medic after she broke off her failed engagement to Chaplain Kyle Sheffield (Teddy Sears) and left Indiana behind for good. But Brett's return to Chicago brings up unresolved history between her and Captain Matthew Casey (Jesse Spencer), which may complicate matters for the co-workers who are likely harboring more-than-friendly feelings.

As Spencer coyly reveals, Brett being back in the Windy City doesn't necessarily accelerate a Casey and Brett romance at the firehouse. At least, not right now. Asked whether things will start to fall into place for the potential new couple, Spencer had a knowing smile as he answered, "We don't know."

"I really don't," he further explained to ET's Cassie DiLaura at the OneChicago press day in Chicago, Illinois. "[Showrunner] Derek [Haas has] kind of written like, we have these characters who have enormous respect for each other. But it's sort of starting to go in a little different direction. They're enjoying each other's company and we actually don't know where it's going. So we're not actually sure if it's going to turn into something personal or not."

The lingering looks and the "two-ships-passing energy" between them haven't been lost on viewers, who are already invested in the unexpected pairing. They even have an official 'ship name, Casett, which Spencer is all in for. ("It's better than Bracey," he quipped.)

Killmer shared her take on the brewing Chicago Fire relationship, cautioning that if Casey and Brett -- on paper, they're a bit of a "weird" duo, she admitted -- were to go there romantically, it wouldn't be without its complexities.

"I also feel like in a strange way Sylvie and Matt have a shared trauma in the fact that both of their Dawsons are gone," Killmer said, referring to the departures of Casey's ex-wife, Gabby, and Brett's ex-boyfriend, Antonio Dawson. "They've been kind of like, 'Sayonara!' Maybe there's a friendship that can happen out of that."

NBC

But friendship may not be on Casey or Brett's minds. While things seem to be simmering for the moment, Killmer entertained the idea of who would be the one to make the first move.

"That's a good question. I don't know!" she said, pondering the question. "It's very much a 'will they/won't they/should they not' scenario of romance."

Killmer's co-star, Annie Ilonzeh, who plays Brett's partner, Emily Foster, had her own idea of how it would play out. 

"Knowing these two, It would be something that is so silly where she would trip and fall into him or he would trip and fall into her. They've got these fun quirks about them that is so unapologetic and sweet," Ilonzeh said. "Honestly, I root for it but I also root for the girl code."

And any notion of Brett's ex-fiance, Kyle, coming back into the picture is out of the question. Killmer confirmed that that chapter in Brett's life is "over." 

"I think she's cut ties," she said. "She doesn't want to drag anybody along. He was so sweet; some people loved him. Teddy Sears did such a good job playing the chaplain, so it was wonderful to work with him. She's back at 51 where she belongs. Her heart is healing."

Haas was candid about the interest for Casey and Brett that seemed to come out of nowhere last season.

"I was surprised at that. The two actors are fun together, so we hinted that there were some feelings going last year," the Chicago Fire co-creator and showrunner said. "I liked the reactions but I was surprised by how much people were talking about it, so I'm not saying it's going to be on the frontburner, but I'm not ruling it out either."

Chicago Fire airs Wednesdays at 9 p.m. ET/PT on NBC.

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