Fox's sci-fi drama Fringe finally delves into the background of fan-favorite character Astrid tonight in an emotional episode called "Making Angels."

Played by Jasika Nicole, Astrid Farnsworth is an FBI junior agent who serves as an assistant to Olivia Dunham (Anna Torv), and over time has developed a friendship with Walter Bishop (John Noble). However, everything changed with the discovery of the alternate universe and the disappearance of Peter Bishop (Joshua Jackson). Tonight, the two universes collide once again in a special Astrid-centric episode in which Alt-Astrid comes to our world to meet Astrid.

"It was actually pretty overwhelming to play against myself -- not only because it was the first huge episode that I had just as an actor, but I was kind of doing double duty as both Astrids," Nicole said in a conference call with reporters. "She's got these really emotional scenes. What would normally take us four or five hours to complete took us eight or 10 hours because we had to do it twice, essentially. There's actually one scene where there are both Astrids and both Olivias and then Walter and Peter. It just took us all day long, and Anna Torv actually pulled me aside and gave me all these tips on how to do it because she's the one who plays her doppelganger most in the show. It was technically very difficult and emotionally difficult, but really, really exciting and totally worth it."

Nicole elaborated on the physical challenge of playing Alt-Astrid, who has noticeably different mannerisms. "Alt-Astrid is much more challenging to play just because she's so physically different than I am," she said. "I didn't even realize that, after the first several months of playing her, I would come home with a headache every single day and it's because she's so stiff in her movements. She's just got these mannerisms that I don't have and my body's not used to them, so it's really difficult to play her. Also, because she speaks quickly, I have to know my lines backwards and forwards because she spits it out really spitfire quick. She's the most fun and the most challenging, actually."

She also addressed Alt-Astrid's motivation to seek out her counterpart, and why Astrid hadn't yet traveled to the Other Side. "I think in the world of Fringe, because they're solving these cases and they're trying to save lives and they're trying to protect these universes from being destroyed, I feel like any ounce of selfishness totally gets taken out of the equation," she said. "Who has time to go cross to another universe and spy on their doppelganger? Hundreds of people are nearing death because there's some crazy vigilante that's out killing them all. I think, deep down inside, this Astrid would have loved to go over there to see her, but it hasn't come up and there's been so much other work to do. Interestingly enough, the reason that Alt-Astrid does come to this universe is for a selfish reason -- not in a negative way necessarily but something personal happens to her that's really traumatic. She doesn't have anyone else to turn to, so her only option is to come here, which I think is pretty incredible. I think it's very brave because I think there's a possibility that this Astrid is too afraid to maybe go over to the alternate universe and see if her alternate has a better haircut than her."

New details emerge in "Making Angels" about the pasts of both Astrids, offering another perspective on the version from This Side. "Alternate Astrid is looking for solace," Nicole said. "She's had a rough life, actually, and this only comes to light in this episode. This Astrid has had such a charmed life compared to the other Astrid. I think that this Astrid has taken that for granted because you never know what other direction your life could have gone down. Seeing this alternate Astrid has probably given her a little more insight into how fortunate she has been and lucky that she is to have the people around her that she does."

Nicole elaborated on Astrid's possible insight by explaining Alt-Astrid's motivation for coming to our world. "[Alt-Astrid's] got Asperger's, so she has some fundamentally different things about her that make her associate with other people and the rest of the world," she said. "I don't think that she necessarily wants to be closed off or wants to be awkward in conversations or social interactions, it's just how she is. I think knowing that there's another version of her out there that doesn't have these difficulties makes her excited, makes her want to meet her and know what her life is like. Because of this traumatic event that ends up happening to one of Alternate Astrid's family members, that's the impetus to bring her to this universe, to find out more about the coulda-shoulda-wouldas. I think that this Astrid needs the Alternate Astrid just to give her perspective on her life. She's been closed up inside this lab for the last four years with this kind of crazy doctor and these really bizarre terrifying things are happening in the world. Then, she meets this woman that looks just like her that could have had the exact same life as her and did not. That, I think, opens up her eyes a little bit to how fortunate she is to have these people in her life. As crazy and dysfunctional her work family is, it is a family that is of great significance to her."

This season, Astrid's work family went through a major change -- even if the character wasn't aware of it. After the events of Season 3, Peter Bishop was essentially erased from existence, leaving the rest of the cast living in a world in which he had never interacted with any of them, something Nicole said greatly affected her relationship with Walter and, once he returned, Peter.

"I think the biggest challenge I went through was this season, actually, when we started this new timeline and I realized that all the relationships were different now that Peter had never been a part of our lives and we didn't know who he was," she said. "Specifically, the relationship between Astrid and Walter was really different because Olivia was more his confidante, his caretaker -- he would seek her out when he was in a bad place and needing help. He didn't really go to Astrid as much, she was more his babysitter than his friend. That was really difficult for me at first because I wasn't sure why Astrid was a part of the team if it wasn't to be Walter's guide and his shoulder to cry on. As the season went on, I realized that because she doesn't have such a close relationship with Walter, I think that she was the only person in this world that was welcoming to Peter. Olivia didn't want to have anything to do with him, Walter didn't want to have anything to do with him, Broyles was freaked out by him and Astrid was the one who was always saying, 'You know, it's okay. He's probably not a bad guy, we’ve got to help this person.' That was an interesting different choice to have made, I just wasn't expecting it. Now that we've had an Astrid-centric episode, I see that her relevance is beyond just helping Peter because you can see that she helps everybody."

According to Nicole, Peter's knowledge of both Astrids came into play in a big way with a huge character moment for Alt-Astrid. "A really important episode for her was when Peter was in the alternate universe and they're trying to figure out what David Robert Jones is up to and there's this moment that was really special -- it's because Peter knows who Astrid is in both universes," she said. "She didn't know who he was at all, but he had the familiarity with her and I feel like he saw her and felt like he knew her and interacted with her as such. There's probably never been a stranger to come up to her and be so friendly and kind and put their hand on her shoulder and let her know that it's okay. She had a reaction to that and I don’t think that had ever happened before in any episode that we'd done. That was a tiny little glimpse into the Astrid outside of her work zone."

Nicole also mentioned her disappointment in leaving the relationship between Walter and Astrid behind following Peter's departure and re-entry into the show, but also excitement at the exploration of Astrid's friendly relationship with Olivia.

"I think the difference in the relationship between Astrid and Walter was a little bit sad to part with," she said, "but what I gained from that besides creating this idea that she's the only person who's welcoming to Peter is this really lovely BFF relationship with Olivia that we never got to see before. I absolutely love that. I think that the fans have been into that too because these are the two women that work together the closest [except] for part of the show when she works at Massive Dynamic and is a bit separate. When we would have all these scenes and you would never see Olivia and Astrid hanging out and talking together, it was like, 'Why wouldn't they be friends?' They're both really smart, they have a very similar difficult job, they work with the same crazy people -- that was a really cool part of this season was seeing Olivia and Astrid hang out together. Now, I have this idea that they probably go out flying twice a month or go to see a movie or something like that. They're with each other so much of the day that they have to depend and rely on each other."

While shooting "Making Angels," Nicole noted the reliance on her co-workers carried over to the real world, which taught her how important the support of the ensemble is.

"I've learned that it's really important to rely on the other people that are around you, on your ensemble," she said. "I never really had a chance to discover that, at least not with this show. It would have been impossible to do those scenes successfully without having the support of the other cast members and crew members on set. They just gave it 110 percent, which made it so much easier to do the work that I had to do. It was truly an ensemble piece with all the crew and the cast members."

"Making Angels" marks a heavy exploration into Astrid's character, something Nicole had been anticipating since the beginning of the series, and the actress was understandably emotional after reading the first draft of the script.

"I think it was a combination of a lot of things that had me so emotional -- not only because it is a really heartbreaking story that is being told with the two Astrids, which is about family and loss and regrets and feeling confidant in who you are and not being able to take that away," she said. "Also, on top of that just the fact that I'd been waiting for four years to do an Astrid-heavy episode. When I finally got it, it was a little bit overwhelming. It's something you've been wishing for for so long and you get to a point where it's never going to happen, I think I had kind of become resigned to thinking, 'Oh, Astrid's never going to get a big episode,' and that's okay. It's been such a lovely thing to be part of the show. Anyway, I finally get the script and it is an Astrid-heavy episode and not only one Astrid, but two Astrids and there's all this emotional turmoil that's going on in the story. It was emotional on a lot of different levels."

Part of that arose from the challenge Nicole faced projecting emotions as both Astrid and Alt-Astrid.

"In terms of shooting it, it was just really, really rewarding and tough. Sometimes, the most difficult things end up being the things that best serve you," the actress said. "It was really, really long hours. I'll tell you, there are two scenes in particular that are extremely emotional where one or both of the Astrids are crying. To try and maintain that level of connection to what these characters are feeling for hours and hours and hours was really exhausting. I think that's one of the reasons the crew and the other cast members were so lovely on those days and so supportive and really, really quiet. It's kind of difficult to be doing something where you're super emotionally invested and then go have lunch and be hanging out and talking with everybody. I know there are actors that are really good at that and I'm just not. I have to stay in this sad frame of mind to maintain that emotional integrity. Everybody was just so enthusiastic and encouraging the entire time."

As for the future, Nicole teased an episode with a major connection to the Other Side -- and a possible encounter between Lincoln and Alt-Lincoln. "We are currently filming an episode that is going to take place mostly in the alternate universe," she said. "There is a lot of Lincoln on Lincoln happening in this episode -- a lot, a lot, a lot. I know the fans will be really excited about that because I personally love the Lincoln so much."

Fringe airs tonight at 9 p.m. ET/PT on Fox.