One of Fox's most-watched shows right now is "The Following." 

The series follows FBI agent Ryan Hardy (Kevin Bacon) as he tries to track down cult leader Joe Carroll (James Purefoy).

Now in season 2, perhaps the biggest additions to the show have been murdering twins Luke and Mark who are both played by actor Sam Underwood. 

To make you believe you're actually seeing double on screen, the show has to film each scene with the actor twice.

Director and executive producer Marcos Seiga tells Business Insider it's been incredibly challenging, but a lot of fun to figure out this season. When he returned to the director's chair for the season 2 premiere he researched every movie with twins for inspiration.  

"I looked at 'Adaptation,' 'The Social Network,'" says Siega. "They clearly used one actor for the twins and I watched those scenes carefully. 'How do they do this? How do they make it feel real?'"

Seiga was familiar with the process having done it before over on the CW's "The Vampire Diaries" where actress Nina Dobrev plays two characters — Elena and Katherine. 

"On that show there was a very simple format where we did a lot of simple split screens and it worked really well and it was effective with that style of show," Siega tells us.

The easiest way to film a split screen is by shooting a scene twice with the same actor. They’ll first appear on either the left or right side of a screen before filming the next scene on the side opposite. Special effects can then make those two separate sequences appear seamless.

Making Underwood's two characters come to life would be a bit more of a challenge. Often, the two are seen interacting on screen together in the same scenes. 

"'The Following' is handheld and it's visceral and it has a little more of a frenetic shooting style," says Siega. "That goes against the controlled environment that you need when you're doing these special effects. So we tried to trick the audience with a double who's an actor who is physically a double for Sam Underwood."

To make it look like Underwood appears on screen as both Luke and Mark, Siega explained how they shoot each scene with the actor and his stunt double, Shan Agish.

"You shoot it where the double does Mark and Sam does Luke and you shoot all your coverage as if you're just shooting two people, as if we were just shooting real twins. We always try not to see Shan's face when we shoot those scenes. 

And then when you turn around and we switch Sam over to Mark you do that sort of complementary thing on the other side and you're able to maintain that handheld feel. It just means that we have to shoot everything twice."

Siega adds it's not that easy sometimes. Other elements can factor in to the length it takes to shoot scenes with Underwood.

"We sometimes have to change Sam and Shan over from Mark to Luke multiple times in an episode and depending on the coverage, the environment we're in, or the weather outside ... it's a big challenge," he says.

In one of the most recent episodes, you actually see the twins come face to face and embrace in a reunion scene.

They touch each other's faces and it looks like they're both side by side.

"What you're looking at there is a classic split screen," says Siega. "Liz Friedlander, who directed that episode, was very specific in the emotional moment of them touching each other."

After filming, they give the visual effects company a break down of what they want done.

"You have to say: This is what we're going to do and here are the pieces we need to make this work," says Siega. "That takes time and that is the hardest part of doing something like that."

"For all the camera trickery and the effects that you put into it, if you can retain the performance and the heart of the scene and the emotion that's really what the audience gets invested in and the scene was about two brothers seeing each other after they thought they may never see each other again and how they wanted to embrace," he adds.

And Siega says Underwood's performance is what really makes the twins believable. Most of the time on set, the actor will have to switch between the two roles instantaneously.

"Because of our schedule, you push him really hard and he doesn't have a whole lot of time," says Siega. "You can't say 'I need a second to adjust.' It's like: 'GO! Just be the other guy.'"

"It's a collaboration and it has its challenges, but the most important part of making it work honestly is Sam." Siega adds. "The fact that you can watch those scenes and you really believe you're looking at two distinct characters is a testament to his ability."

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