Can a single show change the way we watch television? All because of some very active and engaged fans. ABC's nick watt takes us behind the scenes of "Pretty little liars." Reporter: "Pretty little... See More
Can a single show change the way we watch television? All because of some very active and engaged fans. ABC's nick watt takes us behind the scenes of "Pretty little liars." Reporter: "Pretty little liars" is a salacious little show about a high school posse, a missing friend named Alison, and a faceless nemesis named "A" who torments the girls as if from beyond the grave. It stars young women. It's aimed at young women. And -- I think we are changing the way people watch television, and I think these millennials are going to grow up and continue to watch television in this way. Reporter: A bold statement from the show runner. Pll as they call it is the most tweeted about show on TV. The show has Twitter followers in the millions. The stars 10 million followers between them. They are huge on social media. Huge. I think without that aspect our show would definitely not be where it's at. Reporter: Lucy hale, who plays aria, is right now launching a country music career off the back of her pll popularity. ? No one to save you but yourself ? Reporter: Troian belisario who plays alpha Spencer, has been acting since age 3. Her dad created magnum P.I. But boy, have times changed since Selleck's mustache ruled the air waves. Being on social media is very much a part of my job. Part of your job. Yes, it is part of my job. Do you guys respond to the feedback on social media? And does that then shape where you take the show? Oh, immensely. Hanna! Watch out. Reporter: They delve deeper into mysteries. You're too much. Reporter: Created even higher cliff-hangers. Because fans ask for it. It is a giant test group of millions of fans every night. Reporter: Pll is about to kick off its fifth season on ABC family. So what's it actually about? Well, I think that it plays on a very important piece of adolescent psychology, which is that when you are this age it feels like the world is revolving around you. People either hate you or they love you. We're actually the manifestation of those insecurities. They did warn me that you were the smart one. Sorry. We also have lots of hot guys with abs. Does that help? You auditioned for this role, right? I did. Reporter: Sasha pieterse plays queen bee Alison who disappears, they think she's dead, then she returns. I thought you were dead. Yeah, I heard that too. But here I am. As much as our show is a fantasy land, everything is obviously very over the top, at the same time there are some of those things that are still very true to what a teenager goes through. Shay Mitchell plays Emily, who's wrestling with her sexuality. Definitely with my character's storyline, the way she came out and had the support of her friends and family, which is what it should be, I think it was great and it helped -- it did help a lot of people. And there's some racy stuff. Lucy hale's character has an affair. Review it now. So I know I'm on the right track. Reporter: With her English teacher. I mean, that's wrong on many levels. But we checked. I guess it's Pennsylvania where the show is set, it's not. It's not illegal? It's not. It's frowned upon. It's definitely frowned upon. No. I can't take that. Reporter: That plot line, which plays out right here in the teacher's apartment, is straight from the books the series is based on and was kept in as a result of online fan pressure. There's fan pressure and direction over every little detail. The costumes. I put Spencer one time in black-and-white stripes. I didn't hear the end of it because only aria, only aria wears black-and-white stripes. Are you still hating on me possibly? S, Guy one thing about Spencer is she's a huge vintage girl and -- Reporter: Mandy line, head of the costume department, has blax social media star. It's insane what it has done for this entire cast and crew. It's changed our life. To the better? Oh, my god, yeah, to the better. I love this stuff. Reporter: It can get a little creepy says Ashley Benson who plays Hanna on the show and has 4 million instagram followers. Where do you draw the line? You were away for the weekend. I saw you on a golf cart. You're such a -- hello, new "A." Somebody save me. Now I'm uncomfortable. I don't think I can do this interview. And I know everything. "A." What is it about this target audience? I think it's their passion. I think it's their passion for this material, their passion for sort of the social phenomenon that they're building themselves. I think it's really exciting. Reporter: And that is why "Pretty little liars" is changing television. I'm nick watt for "Nightline" in Los Angeles. "Pretty little liars" airs on ABC family, owned by our parent company, Disney. The new season begins tomorrow at 8:00 P.M. Eastern time.
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