"The outcome of this is going to be something that none of them have been expecting and none of them could have prepared for," the executive producer tells THR.

[Warning: This story contains spoilers from the "Isolation" episode of The Walking Dead.]

AMC's The Walking Dead picked up right where it left off, exploring the mental and emotional toll Karen's and David's deaths has had on the group and specifically Tyreese, who experienced the loss of someone close to him for the first time in this new world.

Meanwhile, the flu that claimed Patrick has spread like wildfire -- taking down nearly everyone inside the prison, including Glenn (Steven Yeun). In a bid to treat the deadly virus, Daryl (Norman Reedus), Michonne (Danai Gurira), Tyreese (Chad Coleman) and Bob (Larry Gilliard Jr.) take off on a run for medication but face a massive hoard of walkers and end up losing their car, while Hershel (Scott Wilson) and Carl (Chandler Riggs) look for a natural remedy in the forest near the prison.

The biggest reveal of the hour, however, came in the final moments after Rick (Andrew Lincoln) uses his sheriff skills and pieces together that it was Carol (Melissa McBride) who was the one who killed Karen and David in a bid to contain the deadly illness threatening the group's safe haven.

The Hollywood Reporter caught up with executive producer Robert Kirkman, who penned the episode, to get the scoop on what's next.

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Carol killed Karen and David! Will there be repercussions for her actions?

There's definitely going to be huge repercussions coming from that. I don't think Rick anticipated this danger that was inside the prison to be revealed to be someone they were so close with. It was always, "Hey, we've brought in a lot of people; who knows who it could be." But this is someone who's been one of them for a very long time, so the outcome of this is going to be something that none of them have been expecting and none of them could have prepared for. This is going to go in a pretty interesting direction.

How will we see her struggle with the emotional baggage that comes with this decision?

Carol has a lot going on this season. This is a person who has taken her pain and everything she's lived through and turned it into this pragmatic life she's trying to live now. With training these kids, every moment of her life now is about protecting her loved ones and trying to make sure a Sophia never happens to her again in any aspect of her life. In her making this decision and deciding to go through with this horrific act -- and it's something she's thought long and hard about and has decided that it's something that definitely needs to be done -- being aware of the repercussions that can come from that is part of that. She's very prepared and capable of whatever can come from this, and she's ready for it. This is something that really illustrates the evolution of this character and how much of a different person she is now.

What will Rick do with this information? Karen really is the first person he's lost in this new world and is out for blood.

This is the first big leadership thing that falls into his lap. It's really going to weigh on him because this is the exact decision he's been trying to avoid. Now he's put in a position where he's the only person who can handle this because of the circumstances you mentioned. This is really a big test for Rick. What is he going to do? What kind of decisions is he going to make? Is he prepared to step up into a leadership role again in this instance, or is he going to go a different way?

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Tyreese leaned on her to look after Sasha. What kind of reaction could he have when he finds out it was Carol?

I wouldn't expect it would be a good one. This is not something he's going to be happy about. Just the incident at the very beginning of the episode when he gets into a fistfight with Rick seemingly based on a comment -- he's definitely a guy who's on edge when it comes to this situation. Finding the culprit definitely is going to be something that would set him off. I certainly wouldn't want to be Carol when he finds out.

Michonne, Daryl, Tyreese and Bob are 50-some miles away from the prison with no car. What's that journey going to be like? Do they turn back to the prison, or do they head to the vet not knowing about Hershel's magical berries?

In terms of an actual flu remedy, elderberries -- which you can look up and research just like we did -- it actually does bring a fever down. I think they know that there's not a lot to return back to without these meds, and they're driven on this course. And to a certain extent, they have an entire mega-herd of zombies behind them, so I think they're going to be ultra-incentivized to try to carry this mission out any way possible, which will make for a very entertaining episode four this season.

Can everyone trust Bob considering they still don't really know him?

They kind of have to. We like that mystery. We like the fact that this is the world of The Walking Dead and there is a question of, "Who can you trust?" We've just proven that you can't even trust Carol and they've been with her since the very beginning of this show. Maybe they can trust Bob, maybe they can't, but it's that question that drives things and shows that there are so many dangers to this world. We can keep peeling back layers and finding more things that can hurt these characters or complicate matters for these characters. Even in season four, there are things that you can't foresee. There are always going to be threats moving on the horizon, but these people around you are always going to be presenting the most danger. Maybe the situation with Bob is going to blow up in their faces, or maybe he'll end up being a very valuable member in the group for a good long while. So we'll just have to wait and find out.

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Hershel begins to question if the group might be safer outside of the prison -- will that become something that everyone revisits at some point?

Anyone who's read the comic book series knows that they don't stay in the prison forever. It certainly would be strange if they just decided to stay. That would be a huge departure from the comics and a bizarre move. They are seeing there are dangers to being in the prison. And Hershel is suggesting that it may be wise to hit the road and find some resources that would not see them living on top of each other the way they have been. Whether that's the case, we'll see, but they're currently in no shape to be traveling in any way.

Carl is really ready to shoot to kill anybody. Can he really be trusted with it?

That's a question that probably keeps Rick up at night. But unfortunately they keep getting pushed into situations where they have no choice but to have a gun or you're dead. That's the inner-turmoil that Rick's constantly going to be dealing with. Carl, like all characters this season, is a huge part of this season, and there's a lot of stuff coming with him, and a lot of it does deal with that. These are questions that will be answered throughout the course of the season for sure.

How will Rick and Tyreese's fight change their relationship? Do they have a new respect for each other?

You see in this very episode that it does change their relationship, and it does so in a positive way. These two men are speaking to each other more and are on the same page. The initial thing about that fight is that it showed a lot about Tyreese, but Rick almost felt more guilty about that fight than Tyreese was for starting it. It's starting a dialogue between these two that will probably be on the road to being close friends.

Glenn is infected with the flu and is in quarantine with everyone. Should we be worried?

You should be worried about every character at all times! (Laughs.)

What do you think about Carol being the killer? Do you think her heart was in the right place? Hit the comments below with your thoughts. The Walking Dead airs Sundays on AMC. Stay tuned to THR's The Live Feed for our weekly Walking Dead Dissections with the producers.

E-mail: Lesley.Goldberg@THR.com

Twitter: @Snoodit