Agents of S.H.I.E.L.D.: Fitz-Simmons relationship takes the next step

Spoiler alert! The long-awaited moment FitzSimmons fans have been hoping for since the scientifically brilliant yet socially awkward duo first charmed their way into our hearts on Marvel’s Agents of S.H.I.E.L.D. is upon us.

After more than two seasons of will-they-or-won’t-they, Fitz (Iain De Caestecker) and Simmons (Elizabeth Henstridge) take the next step in Tuesday’s episode. “Their relationship will visibly change forever,” executive producer Jed Whedon tells EW. “Their chemistry is something that was conceived, in creating the characters, as two people who are inseparable. We talked about many different versions of that before casting it — they’re like brother and sister who are always bickering and they complete each other’s sentences, drive each other crazy and are totally co-dependent.”

Adorably, the partners’ dynamic mirrors that of Whedon and his wife and fellow EP Maurissa Tancharoen’s working relationship. “When we first conceived Fitz and Simmons, and we would pitch out those characters, it was something that Jed and I did by finishing each other’s sentences,” Tancharoen says. “It was just something that involuntarily happened. Because of the nature of that relationship, we’ve grown very fond of them. We care very much about this unrequited love story that we’ve built since the beginning.”

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With the show in its third season, the powers that be didn’t “want to string something out to the point of absurdity,” Whedon says, noting that it was easier to keep them apart in the beginning. “When we first conceived the characters, we didn’t think it would go this way. It was only in seeing them play it that their chemistry was so undeniable.”

“For a while, that was the beautiful thing about the FitzSimmons love story is that, to everyone around them, it’s so clearly obvious that they should be together, but to Fitz and Simmons, it seems like the most absurd idea ever to be anything beyond friends, working partners and each other’s best friend,” Tancharoen says.

The trouble now is keeping that spark alive. “We always have to keep it complicated,” Whedon says. “And we continue to do a good job of that moving forward.” Case in point: The rescue mission for Jemma’s off-planet fling may dampen the honeymoon period. “This is the episode where they both will express their feelings finally about everything that’s gone down,” Whedon teases.

Marvel’s Agents of S.H.I.E.L.D. airs Tuesdays at 9 p.m. ET on ABC.