"This is Your Brian On Drugs" -- When Agent Casey Rooks (Desmond Harrington) and his SWAT team skim 80 NZT pills for themselves from the recovery of stolen inventory, it starts an internal investigation that is viewed through the shifting perspectives of Brian's immediate circle at the FBI, on LIMITLESS, Tuesday, Dec. 15 (10:00-11:00 PM, ET/PT) on the CBS Television Network. Pictured (L-R) Mary Elizabeth Mastrantonio as SAC Nasreen "Naz" Pouran, Jake McDorman as Brian Finch, Desmond Harrington as Agent Casey Rooks, Michael Shaw as Agent Mike and Hill Harper as Agent Spelman Boyle Photo: David M. Russell/CBS ÃÂ©2015 CBS Broadcasting, Inc. All Rights Reserved

Spoiler alert! Do not read this post unless you've seen the Limitless fall finale!

Taking NZT proved to be a fatal mistake on tonight's hour of Limitless.

When Brian's (Jake McDorman) dispensary was knocked over, the team was tasked with recovering them. And while they were successful, Casey's (Desmond Harrington) SWAT team skimmed a few pills for themselves. Initally, Casey strongly objected—due to his concerns about how it would impact Rebecca's (Jennifer Carpenter) career—but when she dumped him (via text), he agreed to partake in the drugs.

Unfortunately, the SWAT team then turned on each other while on the drugs—due to suspicions about an affair—and things spiraled from there. Casey and the remainder of the SWAT team took Brian and Rebecca hostage, but Brian was able to steal a phone and send an S.O.S. to Boyle (Hill Harper), alerting him to a gas mask Brian had stored away. Brian was able to talk the SWAT team into releasing gas as a means of escape, but Casey took Brian hostage...leading Boyle to fatally shoot Casey. Even though Boyle insisted what he did was by the book, Brian pointed out that Casey willingly chose not to set off a shot in his final seconds; he already knew he was dying, so he didn't take anyone down with him. And if that wasn't enough, during the madness, one of the NZT pills went MIA...until Boyle found and pocketed it.

So what comes next? We spoke with Limitless showrunner Craig Sweeny to find out what's in store for 2016.

When you cast Desmond Harrington on the show, how much of Casey's arc was already planned out?

We didn't know the end of his arc. Although, we knew we wanted him to get curious about the pill and take the pill, we hadn't thought that through to his endgame. But the seed of it was there.

Once you had him in place, were there any hesitations about taking his character down the NZT road?

You never take the death of a character lightly. Or, at least, I never take the death of a character lightly. There was a considerable amount of thought about it; I guess you could call it hesitations. But it just seemed, as we laid that story out, it kept getting back to that standoff. And it seemed like that was how that standoff would end. We thought it out very carefully before we, so to speak, pulled the trigger.

How much will that doubt about what went down play into how the team interacts going forward?

I think it definitely leaves Brian and Boyle in a fascinating place. What Boyle had to do is going to eat at him, and it's obviously already eating at him, as we saw at the end of the episode.

How will Casey's death impact Rebecca's relationship with Boyle?

I think that it's certainly something they're going to have to deal with. Rebecca is able to view it from the perspective of an FBI agent and understand that Boyle did what he was trained to do. Might it manifest itself in subtextual ways? Absolutely. But formally, I think Rebecca means it when she says Boyle followed his training and did what he had to do.

Rebecca broke up with Casey before he went down the NZT road. Why did you decide to have the relationship end before he turned and then died?

The relationship, they both knew it wasn't going anywhere in the long-term. When we played with the idea of shifting perspectives in this episode, Rebecca's act was always about breaking up with Casey.

Were there any scenarios in the writer's room where Casey attempted to take the shot, to get himself out of that standoff in the end? Or was it important he died with one good deed?

I think what we wanted him to do—and what we explored with NZT, and what we've seen with Eddie Morra's character, as well—is [showing] it benefits the calculating side of your character, and it can overwhelm the human side. That happens very quickly with Casey, and it's something that Brian struggles with all the time. We always wanted to have clarity in that end moment, and know there was a line. There was a strategy to get out of the building, but there was a line that he was never going to cross: which was pulling the trigger and killing Brian.

There is now an NZT pill out there, and the audience knows Boyle has it. How will that come into play as the second half of the season unfolds?

Boyle knows full well the risk of taking NZT, so the idea that he's holding on to that pill is going to gnaw at him. We will see what he does with it before the end of the season.

Is there a part of him that is thinking unselfishly about it? Like, oh, maybe if Brian needs it in an emergency, I have an extra one?

I think there's a part of him that might be—I think the move [to take it] is completely rash and a selfish one. But there is probably a part of Boyle that is justifying it in those ways. Or maybe he has something in mind with his mother, as well. Can he use it to help her in some way? Can he provide her a moment of clarity she might not otherwise have?

The hour also served to show more of Mike and Ike. Now that there's a bit more humanity to them—both for the audience and Brian—how will the trio change going forward?

It was conscious to remind everybody that Mike and Ike are human beings. It was fun to see [that relationship] from their perspective and to explore them a little bit. [Going forward,] they're still Brian's bodyguards, and they still bring him his NZT. As Ike—or Jason—says at the end of the episode, he's gotten used to being called Ike. They've essentially come a little closer together; they've bonded as a unit. Where Mike and Ike resented what they were doing before, now they're more on board because of Brian's respect. And Brian now feels like he has two agents he's working with.

The show has been good at playing with episodic formats. What other format twists are coming up?

We're doing an undercover story that is our homage to the romantic-comedy form. That should be a lot of fun for people. We're exploring an episode that goes into Brian's subconscious in the very near future. We have a lot of ways to shake up the procedural format, and the way we're presenting these stories. We have a lot more fun coming down the pike.

With the rom-com, is that Rebecca and Brian undercover?

It's Brian and someone else.

A male or a female? And is it someone we know, or a new person?

A female. A new female.

And the subconscious, is that the Joshosaurus Josh character you hinted about on Twitter?

Yes, Eddie Pepitone as Josh. Brian finds himself involved in a serial murder case, which is not something we spend a lot of time thinking about on our show, because the world is so unpleasant, and it's not really in the spirit of Limitless. But it is about how, in order to deal with the things he has to do, he retreats into childhood memories in his subconscious.

Looking to the next episode, it features the return of Bradley Cooper again, as well as Georgina Haig's first appearance. What can you say about the hour?

Georgina is [playing] another version of Brian; she's somebody who [Eddie] provided NZT to in the past and has fallen out with and is now at odds with. The title of the episode is "The Assassination of Eddie Morra," and it centers around her attempts to stop Eddie. She believes, in her mind, that he's a bad guy who needs to be stopped. And Brian finds himself caught in the middle with the Morra camp, who wants Piper stopped, basically delivered to them. But Piper is trying to convince Brian he should be on her side.

How much will Georgina play into the second half of the season?

She's going to be in few more episodes after that one. She has an important ongoing role.

What will be Brian's take on her?

It will be impossible to separate his take on Piper the character from the circumstances, which are extremely heightened and dangerous. So he hasn't had time to [sit] down and really think about her. He can see very clearly, at least in her case, that dealing with Eddie Morra has not lead to good things in her life.

This is Bradley's third time on the show, despite his busy schedule. How has juggling that been for you?

Bradley and his team have been very good about communicating availability to us. We're able to plan pretty effectively ahead of time. It hasn't hamstrung us yet.

Looking to Brian's world, how will his family be playing into the second half of the season?

They're a key part to who Brian is. Brian's father now knows about NZT, and is eligible—and is expected—to be drawn into the main storyline.

Limitless returns Tuesday, Jan. 5 at 10/9c on CBS.