<P> Alison made small appearances during the period hiding behind the "A" mask . The DiLaurentis revealed the secret about the twins after a man named Billy Ford was arrested for murdering Alison, saying that they kept the sisters hidden for so long to avoid turmoil in town . Alison, posing as Courtney, someone unknown to the four girls, invited them to spend a weekend at the family home in the Poconos . There, they got together and the girls started thinking about introducing "Courtney" to the group . However, they were trapped inside a room and Alison soon revealed the whole truth about the death of the real Courtney, and about her desire for revenge . Alison then set the building on fire, but the girls managed to escape . When trying to escape the fire, Alison herself got several burns on her body and managed to escape with the help of Nick . </P> <P> Alison then used the following weeks to recover from the fire whilst observing her victims . Nick stayed with her during her healing process, and Alison eventually discarded and incriminated him for everything she had committed, such as the homicides of Ian Thomas and Jenna Cavanaugh . In order to finally get her revenge, Alison wrote an entire diary with untrue stories of the girls torturing her while they were keeping her hostage . The piece was then found by the Rosewood P.D. and the girls were incriminated by the alleged death of Alison DiLaurentis . However, Emily faked her own death, and, while Alison and everyone else thought she was dead, she managed to reach to Alison's lair--which was receiving help from Mrs. DiLaurentis--with police officers . The girls were eventually released from all charges, and Alison was definitely hospitalized . </P> <P> Actress Sasha Pieterse was cast as Alison DiLaurentis in December 2009 . Alison was the queen bee of her school and the most popular girl before her disappearance . Although she cared for her friends, she enjoyed to use their strongest secrets against them to that they could, according to her, stay together . Charming and manipulative, and highly intelligent Ali was skilled at finding ways to get back at everyone, and many people hated her . After the reveal of Alison's fate in the ending of season 4, Pieterse confirmed that DiLaurentis' personality had drastically changed; she has to "figure out who she is . She's been hiding for so long, and she's a victim now ." "Alison's a totally different person, so to see the way they treat her is going to be very interesting and to see the way that she gets affected by it is also going to be very interesting," Pieterse said . </P> <P> Alison DiLaurentis is introduced as a very popular girl from the suburb of Rosewood, Pennsylvania, and the leader of a clique composed by her, Spencer Hastings, Aria Montgomery, Emily Fields and Hanna Marin . The first minutes of the pilot episode show the five friends reunited in the barn - turned - loft of the Hastings family during a night of summer vacation . In the midst of night, Hanna, Emily and Aria wake up to see Spencer arriving back to the barn after leaving to somewhere . Spencer naively announces that she has heard a scream of Alison, seemingly shaken due to something . Shortly after, Alison is pronounced as a missing person . One year later, the friends reunite and start to receive threatening text messages from someone unknown using the alias of "A ." During the entire first season, Alison appears in flashbacks . In these flashbacks, Alison is shown as a ruthless, despicable, two - sided teenager who uses the others' secret against themselves . </P>

Where has alison been on pretty little liars