Pretty Little Liars

Pretty Little Liars star Sasha Pieterse has two words of advice for viewers watching the Halloween episode: Pay attention.

"It amplifies evidence and also a new suspect arises," Pieterse says. "This person is someone you haven't really thought of before, or if you have, there's not much evidence to back it up."

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The upcoming prequel is told from the point-of-view of Alison (Pieterse), whose mysterious death is the basis for the series. For the first time, viewers will see Alison outside of flashbacks. "It shows her vulnerable side ... and her first encounters with her enemies," she tells TVGuide.com.

Pieterse, 15, elaborates: "Pay attention [to] the first opening scene. Keep looking at their surroundings. We also see Alison's room for the first time. Check out her room for some interesting things in there."

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A key element is Jenna's arrival to Rosewood. An interesting scene between the two shows that Alison once recruited Jenna (Tammin Sursok) into her group. "Alison always hated Jenna, but you don't really know why and this ties it all in. She wanted her on her side because she realized Jenna had potential," she says.

Pieterse says the information uncovered in Wednesday's episode will be addressed when the show returns for the second half of Season 2 in January.  "Now the viewers know things from Alison's perspective that [Aria, Spencer, Hanna and Emily] haven't figured out. It escalates from there and they will soon catch up with the viewers," she says.

Watch a clip of Alison and Jenna's first meeting from the episode airing Wednesday at 8/7c on ABC Family.