The Insider.com

Watching Cassie Blake decide if she's a good witch or a bad witch has transformed The Secret Circle into a hypnotic hour over the last few weeks and tonight's episode promises to ratchet that tension up three notches as Circle travels back in time to reveal exactly what happened during the fire that forever changed their parents' generation.

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To find out what awaits in Witness, I caught up with star Britt Robertson and learned that while tonight's episode won't make Cassie/Adam fans very happy, it will answer almost all of the season-long questions -- except, Who Is Cassie's Sibling? That reveal is even being kept from the cast!

Insider.com: First, watching Cassie blow the top off that coffin was one of my favorite moments so far on the show.
Britt Robertson: Wasn't it awesome? As an actor, it's hard to find this kind of role on television that gives you this kind of material to do and this show is giving me two things to play at all times – it's a challenge but it's also what every actor dreams of. I'm super happy we're going for it!

Insider: Last week's episode ended with Cassie in the middle of a literal Jake/Adam triangle - where does tonight's episode take those relationships?
Britt: It's a really cool episode for that reason. With Fire & Ice, you really got to see Cassie and Adam fall for each other all over again. Now that they've moved on from the Cassie/Adam/Diana triangle, she's getting to explore her feelings for Adam. But in this episode, you'll see her with Jake more and all the feelings that come up about her dark side, which is what he really represents. It'll be a great episode for Jake/Cassie fans, not such a great one for Cassie/Adam fans – but it sets up that triangle even more for the following episode so you pick a side and decide who your main man is [laughs].

Insider: Who do you prefer Cassie with?
Britt: I kind of love the idea of destiny and love at first sight. That whole connection you saw with them in the pilot was so great. I'm kind of on Team Adam, but I think Jake brings out a really cool side of Cassie, so from an actor side, I like how dangerous he makes her.

Insider: What about the introduction of John Blackwell, her father -- what does that bring out in Cassie?
Britt: It's a cool, different dynamic than you've seen with Cassie. I think it'll just add more depth and emotion to the character that the audience hasn’t seen. You'll see the 16-year-old girl that we don't often see in her because she has to deal with so many supernatural occurrences. I can't exactly reveal how he comes back – there's the idea that he is alive and real, but he could be another flashback. The audience will just have to stay tuned and see!

Insider: A few episodes ago, it was hinted at that Blackwell also fathered another member of the circle. Do you know who Cassie's sibling is yet?
Britt: I don't know, I still have no clue. I don't think that even when they reveal it, they'll tell us – we'll probably shoot multiple takes of it being different names so we can't crack and tell anyone [laughs].

Insider: Aside from the relationship stuff, what else should fans look forward to with tonight's episode?
Britt: It's unlike anything you’ve ever seen from us. We go back in time to answer all these fan questions. There are some big reveals, which makes it a game changer for almost every character and it's a great jumping off point for the rest of this season. Next week, there's some great stuff with Faye & Cassie, Cassie & Jake and Adam & Diana as the circle relives all these dynamics that we set up in tonight's episode.

The Secret Circle airs Thursdays at 9 p.m. on The CW.