The entire writers’ room thought a lot about this and part of it was actors’ availability. When you don’t own certain actors and they’re not series regulars, you never know if they’re going to be available to you. There were times when I loved the idea of Wren being A.D., Melissa being A.D., a combination of those two, but ultimately we never knew if we were going to get those people in these last 10 episodes and that sort of started discussion that A.D. really needs to be someone who’s a series regular and fans were always wanting it to be one of the girls … which makes no sense to me because they really would be torturing themselves — the mythology would never work. So that’s when we thought, well we love twins on this show and making it a twin of a PLL is sort of having your cake and getting to eat it too, so it’s almost a “pretty little liar.
— Marlene on choosing A.D. (AKA WE could have had WREN/ MELISSA AS A.D! OMFG.)