Agents Of S.H.I.E.L.D.'s Iain De Caestecker & Elizabeth Henstridge Discuss FitzSimmons' Relationship

On tonight's episode of Marvel's Agents of S.H.I.E.L.D., one of the show's longest running relationships, and a fan favorite, will be changed forever.

The feelings that Agents Jemma Simmons and Leo Fitz have felt for each other, and took so long to properly express, will finally reveal themselves in a visible way. But to say this relationship is complicated – Simmons spent six months on an alien planet, and fell in love with a stranded astronaut – would be a bit of an understatement.

The Marvel's Agents of S.H.I.E.L.D. stars who play FitzSimons, Iain De Caestecker and Elizabeth Henstridge, spoke to ComicBook.com about what their characters are thinking and feeling going into tonight's episode.

When you two first stepped into characters as FitzSimmons, did you have any sense that it would become a romantic relationship, or did you believe it would remain platonic, or even familial?

EH: I think there was always a possibility of that, but we spent all of first season saying, "Oh, they're like brother and sister, or a super old married couple." I never thought they would actually kiss, or get together. And who knows if they will get together. But I feel like we spent so long trying to convince everyone that they were just brother-sister, so this caught us off guard I think.

ID: I concur.

Elizabeth, the last time I spoke to you, you said that Fitz and Simmons were putting their feelings and the complications of their relationship aside so that they could focus on the task at hand, that being bringing Will back from the other side of the monolith. How has that been working for them so far?

EH: [Laughs] Obviously not very well! I think just the way that Fitz reacted was, every time they'd work on getting Fitz back – and Fitz is so brilliant about it – that she just fell for him more and more. She never stopped caring about him, so it just kind of compounded those feelings. I think she was in love with two people. She's very confused, but she's explored that area with Will, and she's never explored that area with Fitz, so I think you've got to a point where all the chemistry was just bubbling over and they were kind of taken over by the moment. But it's still a very complicated situation.

Iain, Fitz seems to be trying to do the right the thing by trying to get Will back, at least outwardly, but there must be quite a bit going inside his head. What's motivating him to action at this point?

ID: I think both Fitz and Simmons have always had quite a good moral compass from the get-go, and its probably quite a confusing situation because no one's done anything wrong. There's no villains in this kind of story, it's just circumstance and it's complicated. I think he's trying to do the right thing. I think it's easier for them to do that right now, so they don't have to properly confront it, though obviously it will come to a head at some point. Obviously it can be difficult for either of them. At the same time, he's still an agent of S.H.I.E.L.D., and he has a responsibility to do the right thing.

airs Tuesdays at 9 p.m ET on ABC.