Originally slated for just a one-off appearance, Chelsey Reist has since become a series regular on The 100, with her character Harper McIntyre becoming an integral part of the ensemble.

Between her burgeoning relationship with Monty Green, her "badass" status as an Arkadia guard, and her recent falling out with main character Clarke – all while the world is literally ending around them – The 100 Season 4 is gearing up to be an an exciting time for Harper.

We spoke with Reist about learning more of Harper's backstory, how she'll handle the rapidly approaching apocalypse, and why she became an actress. The actress also dished on who she'd 'ship Monty and Harper with if they weren't paired with one another, as well as one of her all-time favorite The 100 scenes to shoot.

Check out her thoughts below.

Harper had a big moment on The 100 Season 4 Episode 4, where we found out that she wasn't going to be on Clarke's list because of her dad's genetic illness. The list issue was resolved, but is that illness going to come up again at any point this season?

Chelsey Reist: We are going to learn a lot more about Harper this season than we've ever known. I am lucky enough to be a series regular this year, so that means a lot more Harper screen time.

We’re going to learn lots about her, about her past, and about how she feels about the world ending. I think we're gonna have some surprises about how she handles that.

What kinds of surprises?

I guess I can tell you that even I was shocked when I read the script. I thought, "Whoa! OK!"

So Harper's going to be making some big moves this season, then?

Different moves! Different than I would've expected. I had to find how Harper would get to certain decisions. It felt, like I said, like a surprise to me.

But once you do the script work as an actor you can find, wow, of course she would react that way, and you find where it came from. But it's something that I really connected to on a personal level, so I'm excited to show the world.

How will not being on Clarke’s list affect Harper's attitude this season, her friendship with Clarke, and her place within the larger main group of characters that we follow?

Harper's always trusted in Clarke and her ability to make decisions that have been heavily weighed upon. We know Clarke doesn't take anything lightly, and that's why we've grown to love her. So, for that to have happened was pretty shocking to Harper, because Harper thought, you know, she's in Clarke's good graces, they were friends.

But not only did Clarke exclude her and Monty from her list, but she’s now going into Harper's files and reading about her father and this possible genetic disease. That pushes Harper away, for sure.

So we're going to see the downfall, not just with Harper and Clarke's relationship, but everyone's relationship with someone who tries to play god. Things are definitely set in motion.

Did you know all along that Harper would eventually end up with Monty? Did it naturally develop, or was it decided on at any specific point?

It's funny to hear that, as you just said, it kind of started at the end of last season, because I feel like it's been growing for so long! Well, maybe that's just the decision that I made as an actor before we ever saw it onscreen.

But I feel like it started developing when, in The 100 Season 2, they were trapped in Mount Weather and Harper was being harvested for bone marrow.

They were kidnapped together and I felt an allegiance between Harper and Monty. They've been through something so traumatic. So I feel like that's where it started budding.

As with anybody who goes through a trauma with someone, you start to have this innate understanding of each other, this unspoken trust and understanding.

I think Harper and Monty offer something beautiful on The 100 where things can just pause and you can just breathe. There was that love scene where Harper said, "This is the first time I've ever felt safe." And that's a huge thing on The 100!

I feel like we haven't really seen that for a long time. But with Harper and Monty we're seeing it now, and a little bit with Kane and Abby, too, we're finally having this reprieve.

I'm a fan of their relationship. I'm so happy to hear you say that other fans are too because I guess some said it was a little unexpected. But to me, it's just a natural growth, and this season we will see more of their growth and more of what happens after a relationship starts.

And I’ll leave that up to your imagination!

If Harper wasn't with Monty, who else would you have paired her with?

[laughs] Harper, I feel like has so much love to give. She's so welcoming and so loving, and she's obviously a tough chick, man – she’s kicking butt, she’s a guard, she’s defending her own. But she does have a soft side.

I'd like to think eventually she would find love with someone else, but if it weren't her and Monty locked up in Mount Weather, and Monty slowly nursing her back to health and watching over her, I'm not entirely sure she would've loved. I think she would've gotten more callous. More like Octavia, you know?

But if I had to pick, it'd have to be someone who would be willing to fight the battle but is also sensitive enough to remember that she needs to be taken care of privately, soooo...

I don't know, I feel like I'm starting wars with this. [laughs] I’m sure we can all imagine who’s the strong character who's also sensitive...

Are we going to have to start shipping Barper?

[laughs] Barper!

Hashtag Barper. Yeah, it's a pretty bad 'ship name.

No, that can't ever happen. That's why it can never happen.

Right, because of the name.

And Monty's so smart and very grown up. I feel like he's got this attitude that's very adult, that's very responsible, and he can respond to situations in a very responsible, calm way, so maybe for a relationship with Monty – maybe Abby? Somebody who would be stimulating to him intellectually. Maybe Raven?

Do you have a favorite episode of the show in general, or even a specific scene that you had to shoot?

I do, but they're yet to come, so I'm trying to think of what has passed. My go-to, when I think about why I love The 100, I always go back to the scene in Mount Weather when they’re stealing Harper’s bone marrow. I've always been a fan of horror movies, of the dark genre. And I feel like that's a moment when The 100 really went there, and I got to be part of it.

Strapping a human being down, kidnapping them from the group and immobilizing them, and stealing their bone marrow is so brutal. I had a lot of fun filming that. When we were filming it and Harper's just strapped to the table, they strapped me down and in between scenes they can come and release you.

In the scenes, I said, "Just leave me here, between the scenes, just leave me. [laughs] I wanna feel how horrible that pain would be."

And it really gave me this horrible sense in my stomach, being strapped face down on the table and everyone's moving around and resetting cameras or talking or laughing or talking about what's for lunch and I'm just left there on my own.

It gave me this really... creepy feeling that's kinda stuck with me. I think that's what's so good about The 100 – it gives you these creepy feelings that stick with you.

That scene was really dark.

Here's what it is: It's no secret that growing up, I had depression for a very long time, and I consider the ability to feel that deeply my gift. And that's why I turned to acting, because I can empathize so deeply with other characters and other people.

That depth of feeling that has always come so naturally to me. I like to drag the viewers who aren't comfortable in that zone, I like to drag them there. So, I think that's why I like that dark scene.

Of course, at times [depression] doesn't feel like a gift, and it’s very crippling. But on the other hand, it has been my gift, and I found a way to use it. I remember stopping to think what are my gifts, what is my gateway to the world.

That's something that's always been important to me, to make people feel things they're too afraid to feel. I've had a fantastic opportunity to do that on The 100.

If you could have The 100 cross over with any other show on The CW, which one would you choose and why?

iZombie! The genres are so completely different that I would love to see how the writers would do that. I feel like also something that's dark, but kind of funny? Like Fargo. That kind of feeling, like “I shouldn't be laughing at this!”

Also because I have friends on iZombie – Rob Buckley [who plays Major on iZombie] is one of my dear friends, Rahul [Kohli] is so fun, they're really fun people to be around. Let's pitch that! Let's make that happen!

Can you preview Harper's arc this season for us? Tease us.

Some of my favorite moments that are coming up are unexpectedly jovial. There are certain moments coming up that are quirky, and I find that interesting. And I find that important.

If you think about the world ending, how would we react? A lot of us would lose our S-H-I-T, a lot of us would freak out. We'd all react in different ways. There's just so many different reactions, and I think the show is going to show that.

We're gonna have a chance to see, as I said earlier, some unexpected reactions from characters. And I think that will be fun to watch, from characters we've known and loved for several seasons.

I'm loving the show as it releases, 'cause I'll be reading the script but then watching every Wednesday, that's just a different experience – like when Tree [Adams, the show's music composer] comes in and adds the music.

Everyone's craft really comes together to make it so much more impactful. From what I've read of the season, I love it. And I know when we add those multi-dimensional layers, it's going to get even better.

I would tease that this is a very memorable season. I think it's going to touch a lot of people. Again, we've had some very touching scenes and moments that, good or bad, have left people with an opinion on The 100, and I think we're going to have another season of unforgettable feeling.

We've always fought each other, right? We've fought the adults in the sky, and we’ve fought the grounders,  and we've fought each other. But now, we're all fighting the world. That's one giant enemy.

So we're going to see relationships change, we're going to see alliances change, and, yeah, it's just good. Can you tell I’m really excited about it? [laughs]

Be sure to tune in for The 100 Season 4 Episode 6 "We Will Rise" when it airs on Wednesday, March 15th at 9/8c on The CW.

Caralynn Lippo is a staff writer for TV Fanatic. Follow her on Twitter.