Credit: ABC Family

On tonight’s episode of Pretty Little Liars (Season 4, Episode 19: “Shadow Play”), we get a glimpse into Spencer’s drug-fueled mind. Her search for “A” is bringing her closer to piecing together the puzzle, but when her lack of sleep leads to a black-and-white delusion, it’s going to be difficult to separate what’s real and what’s just a figment of Spencer’s wild imagination.

But why is Spencer pushing herself to the brink of a mental breakdown (and a trip to rehab)? Wetpaint Entertainment caught up with Troian Bellisario at the New York City signing of her Seventeen cover last month to talk about detective Spencer’s motives.

“With Alison, it was never a cut-and-dry best friendship,” she told us. “Each one of the girls has an incredibly complicated relationship with Alison, bordering on blackmail, and especially Spencer. Spencer was always in competition with Alison, and in fact, there were several moments where Spencer was ready to completely shake Alison off as a friend. Then, Alison disappeared, and that was a big thing for Spencer to come to terms with. How do you feel when somebody you wished were dead might actually be dead? So, what happens to Spencer is that she thinks she’s the one who has to get [Alison] back because she was the one who pushed her away and didn’t help her that night. She still doesn’t remember what fully went on the night Ali disappeared. So Spencer really takes it on as her responsibility to be the detective and make it safe for Ali to come home.”

Credit: ABC Family

So it sounds like Spencer’s investigative benders are being fueled by her guilt. We know Spencer has been keeping a “huge” secret from the Liars, and it looks like something happened between Spencer and Ali the night Ali disappeared. Luckily for us, Season 4, Episode 19 will give viewers even more pieces to the puzzle. Did Spencer push Ali away for good that night? Or worse, did Spencer have anything to do with Ali’s disappearance? After all, there’s a reason Ali doesn’t trust Spencer.

“There’s definitely a lot of guilt. Spencer always has a big secret, but this is a big one,” Troian told us. “And, this is what’s so great about the show. They are unafraid to keep something like this for almost four seasons. We’re going to learn what actually happened the night Ali disappeared, and it’s going to be so fulfilling for fans who have been waiting years for it to all come together. The fans are going to find something out about Spencer this season that the girls didn’t know anything about. It’s going to be a very interesting turn in her life.”

Spencer’s secret won’t just affect her relationship with the Liars; it will also come between her and Toby. Spencer and Toby have been through so many ups and downs this season, but it sounds like Spencer’s secret might drive them apart for good. And we didn’t think anything could be worse than Toby’s “A” team reveal! With Spencer focusing 100 percent of her energy on nailing down “A” and finding Ali, it leaves little room for romance.

“What really comes between them is Spencer’s secret because as she begins to go further and further down the path of trying to find Alison, she pushes Toby further and further away,” said Troian. “He doesn’t know how to deal with losing her again. They ultimately got this real closeness when they both joined the ‘A’ team, and then came back and rekindled their love, and now she’s really pulling away and putting Alison before him. She becomes quite obsessed with finding Ali and unmasking ‘A,’ which leaves little room for Toby. So, their relationship goes through a lot this season.”

Clearly, Spencer is headed for another breakdown, but we hope her big secret doesn’t alienate herself from the Liars. It sounds like she’s going to need her friends when this all comes to light.

Catch the next episode of Pretty Little Liars on Tuesday, February 11, 2014 at 8 p.m. ET on ABC Family.

Crystal Bell is an editor at Wetpaint Entertainment and our resident fangirl for all things Pretty Little Liars. Follow her on Twitter and Google+!