Its called acting -

So it seems as though the two possibly meanest characters on PLL are actually not so mean, guess they call it acting for a reason. Noel Kahn and Mona Vanderwaal appear on the show as liars, and trouble makers but in reality these two are total sweethearts! Recently Janel Parrish and Brant Daugherty posed for Adam Bouska's NOH8 photo shoot, to express a silent protest they were participating in. The NOH8 campaign is described as a "charitable organization whose mission is to promote marriage, gender, and human equality through education, advocacy, social media and visual protest" by the NOH8 campaign website.

These two took amazing pictures, and in many of them they have duct tape on their mouths and "NOH8" tattooed on their cheeks, they are very moving pictures and I suggest you check them out by clicking on the link below, and let me know what you think of them!

Also a few months ago, another PLL actress took part in the NOH8 campaign, and that was Shay Mitchell who plays the role of a lesbian in the show, so she would of course be supporting the cause, not for her personal reasons but because she gets the feel of how her character feels about herself, she puts herself in her characters shoes at times, she has said. 

More NOH8 pics can be found:
http://www.noh8campaign.com/photo-galleries