Ever wonder why some TV couples just seem to go together? Of course, good actors can portray romantic interest, but to convince the audience of a real connection, you need something called chemistry, and Kara Killmer and Jesse Spencer, who played paramedic Sylvie Brett and firefighter Matt Casey on Chicago Fire, respectively, had the definition of chemistry. After finishing her character’s storyline with a wedding to remember, Killmer revealed the secret to her and Spencer’s on-screen sparks.

How Kara Killmer and Jesse Spencer generated chemistry for Chicago Fire

“We really just spend a lot of time laughing and joking around. [Spencer] is such a clown in real life, and so delightful, you know?” the actress told NBC Insider. Though the whole One Chicago franchise is about as dramatic as it gets, the set is a ton of fun. “There’s a lot of laughter that goes behind just about every single thing that we film,” said Killmer.

Killmer talks Casey’s return on Chicago Fire Season 12

Though the Brettsey wedding was Brett’s send-off, Casey had already departed Station 51 (Spencer was last season in the finale of Season 11). And the reunion was a warm one. “It was such a treat to get to have him back for this episode. It was like coming home, you know? We picked up right where we left off. It was really sweet getting to have him back for this episode. It was like he never left,” Killmer said.

And what an episode it was. “I think one of the things that’s refreshing about this episode is in their vows; they are finally getting to say to each other face to face what everyone has understood, but it’s kinda been left unsaid,” Killmer explained. “You don’t frequently get to hear them talk about how much they love each other, and so that was something that was very satisfying about filming the wedding is that you finally get to hear them say to each other what everyone has been seeing and feeling and thinking for, you know, the last, like, five years.”

Is Sylvie Brett gone forever on Chicago Fire?

While there are currently no plans to bring the character back, a cameo isn’t off the table. In her goodbye post to Killmer, co-star Miranda Rae Mayo (Lieutenant Stella Kidd) wrote, “I know this won’t be the last time we see you at 51. See you soon, sister.”

Plus, Killmer herself told NBC Insider about the possibility of returning to the show, “The door is wide open. Any time they wanna call, I will be there. It is a fantastic show, great people. I will come back any time they want.”

— Reporting by Jessica White