Plus, see an exclusive clip from the midseason finale

By Devan Coggan
Updated December 15, 2015 at 06:45 PM EST

Since joining the Limitless crew, SWAT team leader (and Rebecca’s secret boyfriend) Casey Rooks has been a valuable asset, teaching Brian martial arts and taking down multiple members of the FBI’s Most Wanted list. But the last episode of 2015 finds Rooks (Desmond Harrington) on the wrong side of the FBI, as he and his team recover a batch of stolen NZT and secretly skim 80 pills for themselves. EW has an exclusive clip from the episode above, and we talked to Harrington about why Rooks would go rogue.

“You see from the beginning, he had an amount of curiosity about what was going on with Brian, what Brian was on,” Harrington says about Rooks’ interest in NZT. “I always felt that he probably would be willing to do it, if the circumstances presented themselves.”

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Not only is Rooks secretly hoarding an NZT stash, but things are also rocky with Rebecca, who is portrayed by his longtime Dexter costar Jennifer Carpenter. In fact, it was Carpenter who originally reached out to him to see if he’d be interested in signing on for Limitless.

“I’ve always remained friends with her, so there was no doubt,” he says. “It was just perfect to work with her again. After a certain amount of time, you know want to work with certain people you’ve worked with, regardless of whatever it is.”

Limitless airs Wednesdays at 10 p.m. ET on CBS.