The Following's Sam Underwood: "Mark Has a Huge Amount of Power Over Luke"

It's been double trouble for Ryan Hardy (Kevin Bacon) and the FBI on The Following, thanks to the terrifying twins Mark and Luke (both played by Sam Underwood). But despite what we've learned so far about the diabolical duo, viewers shouldn't assume that Luke is the one who's calling all the shots, according to Underwood.

"On the surface, [Luke is the leader], yes," Underwood tells TVGuide.com. "But Mark also has a huge amount of power over Luke. You see that in the first episode when Luke is about to go all crazy on Carlos. Just before he's about to kill him, Mark kind of reins him in. ... He has the ability to be able to do that with Luke because of their relationship. Luke is the front-man of the band. He's the Paul McCartney. But Mark is the Ringo Starr. He's there keeping it all together, even though he's not at the very front."

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That dynamic doesn't necessarily mean that Mark poses the greater threat, however. After Luke watched his cohort — and lover? — Giselle (Camille De Pazzis) get stabbed to death in last week's episode, it's safe to assume that he has a renewed sense of motivation to kill Ryan.

"Purely based on the fact that Luke is the more impulsive one, doesn't always think before he acts, and is kind of a bit more of a rebel without a cause — or a rebel with a cause — I would say Luke's the more actively dangerous one," Underwood notes. "In a very broad sense, Luke loves to be seen, heard, celebrated for what he's done. He loves the spotlight, whereas Mark is a lot more reserved and thinks before he acts."

But not all of Mark's actions are calculated. Viewers learned last week that the more reserved twin suffers from haphephobia — meaning he can't handle being touched. We're guessing that doesn't bode well for his inevitable clash with Ryan, or for his potentially budding romance with Emma (Valorie Curry), whom he idolizes.

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"Mark believes that Emma would be a great addition to the little army that they've created," Underwood says. "He sees it as a good thing for the team. But also, Mark has a bit of a celebrity crush on Emma. Whether or not he'll admit it to his brother or anyone else, that's definitely part of his motivation [for bringing her to the house]."

Fans will learn more about Luke and Mark's backstory, as well as their relationship with their "mother" Lily Gray (Connie Nielsen) in future episodes, Underwood promises. Though it remains to be seen whether Joe Carroll's (James Purefoy) new "family" will provide a safe haven, or if the group's closeness is actually a huge liability.

"Mark, Luke and Lily have such a strong unconditional love, and absolutely that adds stakes for them," Underwood notes. "Any relationship that's established in this show is in peril of being destroyed. I think the safest way to be on The Following is to be a complete loner."

And with Emma now in the mix, the power dynamic between Mark and Luke will become even more complicated, Underwood assures.

"You put three sharks in a tank together, I don't think they're always going to get on swimmingly," he says. "These are all highly dangerous, intelligent individuals, and no one trusts each other. ... It's going to play out in a very interesting way."

The Following airs Mondays at 9/8c on Fox.