On the heels of appearing on several episodes of Chicago Fire , the busy Sarah Shahi (Life, Fairly Legal) will make her debut this Thursday night on Person of Interest, kicking off a new recurring role.

Shahi plays a character known as Shaw, a tough-as-nails member of a secret paramilitary organization, who makes her presence known to Finch (Michael Emerson) and Reese (Jim Caviezel) in a big way. Shahi’s debut, in the episode “Relevance”, also marks the directorial debut of Jonathan Nolan, Person of Interest’s creator – and, of course, the co-writer of The Dark Knight and The Dark Knight Rises, with his brother, Christopher.I spoke to Shahi about her mysterious new role on Person of Interest and what it’s like playing such a skilled and deadly woman.Yeah, she’s the ultimate badass. I went in to go do some ADR, and I saw what I was doing. I was like, “Holy s**t, I am not that cool! This character is f**king cool!” She would definitely not hang out with me. It’s funny, I say if Jason Bourne and Catwoman had a child, this would be her. She is sexy, she is deceptive, she is dangerous, she is flawed. And without being a superhero, she’ll just use anything that she can to get the upper hand in a situation. She’s completely fearless. There’s nothing really that holds her back. She’s almost like a machine in the way that she operates.Well, I can’t tell you much. If I told you, I’d have to kill you -- and by the way, now I can, because I’ve gotten training on that. I can take a newspaper and make it a lethal weapon. But what I can tell you is that she is a completely fearless operative in the most secret, covert, paramilitary branch of the government that there is. Just like Reese and Finch, she also tracks terrorists before they have a chance to execute anything. So these characters are kind of living these parallel lives in some way. Although, Reese and Finch don’t really handle big terrorist acts, they’re on a smaller scale. They’re kind of doing the same thing, she just does it on a bigger scale, and at some point they intersect.This is a multiple-episode arc, and I can’t imagine my first episode being directed by anybody else, because this really is kind of inside his head. This was a character that he wanted to bring on board. As a director, he was wonderful to work with, and he knew exactly what he wanted. He didn’t waste time doing anything that wasn’t necessary or anything that he couldn’t fix later. You know, it’s a very high action, fast moving show. This episode in particular, the moment it starts, it really just doesn’t let up until the credits role. It’s kind of intense. So yeah, they move very quickly on the show. The action was movie caliber stuff that I and my stunt double were doing. It’s very impressive. Again, I can’t imagine Jonathan putting this in another director’s hands.Only things that I needed to know. Some of it they were forming as we were going along. I just know the very broad strokes of what’s going on, but nothing that’s too detailed or would give anything away, even for my benefit. Honestly, as an actor, all I need to know, the way I kind of look at a scene, is like a puzzle. There are certain puzzle pieces that are bigger than others, and all I need to know is if this is going to fit here to make this part of the puzzle work. So I don’t need to get everything all at once. For me, sometimes the mystery of it is good, and it allows for me to play with different choices in the moment, which as an actor can be very liberating. You don’t know exactly what you’re playing to, but different things happen with each take. It’s very interesting, the different layers that can come up. But again, I know the very broad strokes of what I need to know to help form the character in the next few episodes. Specifically where it’s going, I couldn’t even tell you that.Well, it was definitely a crash course in how to become a killer. I had less than seven days pretty much, divided between New York and LA, to learn different forms of military weapons. I had a wonderful trainer that came in, James Deever, to help me. He had 25 years in the marines, and he’s amazing. He came in and taught me a bunch of different stuff. Then I worked with their stunt team. I have to say, for it being the first time I’ve stepped into a role like this, that side came out very quickly and very naturally. I just hope that when people watch it, they believe it, because it felt pretty damn good. I’ve made my husband a little bit more nervous, I’ll say that. He’s a little intimidated.There was one stunt that was not safe for me to do, and I knew that going in. I’m the one where they say, “We need someone to hang from a building,” and I’m like, “I’ll do it!” I want to do everything. But this, I tried, and they were like, “Absolutely not.” And when the stunt was over, I could see why they did not let me do that one. But for the most part, anything that you’re watching is me doing it. I was able to pull all of it off. Every time I pulled something off, I was like, “Holy s**t, did I just do that? And it looked good? You guys believe it?” They were like, “Yeah, you sold it. You did it” “Really!?” So as it was happening, I couldn’t believe it was happening.Michael Emerson is just a prince. There’s something about him. He’s so sweet. I don’t know how to describe it. There’s something about him that’s a bit royal. My time with them, ask me this two months from now and I might be like, “Oh, he’s a dick!” [Laughs] But I don’t think that’s going to be the case. No, they’re both incredibly giving actors. This was my first episode, and obviously the new kid on the block doesn’t want to press her luck, you know what I mean? So I respected their boundaries, and they respected mine. We all worked really well with each other. When we could chat in between takes, we did. But our stuff is pretty intense, so there wasn’t really a lot of chatting this time around.I did, I did! [Laughs] I don’t think there was one person on the show whose ass I didn’t kick -- or at least try to kick.No, I can’t, because we want it to be a surprise.Yeah, as of right now, I’m done with that show. That was just a five-episode thing, and I just had to make out with Taylor Kinney each week, so how I could I turn that down? It was a pretty easy job. But it was fun, and we all got along so famously well. But they were kind of sad that I was going over to Person of Interest, I think, because there were plans to have Renee back with Severide and see where else that character was going -- because it does seem like they still have some unfinished business. So we’ll see what happens with that.

Sarah Shahi makes her debut in Person of Interest on Thursday, February 21st at 9pm ET/PT on CBS.Eric Goldman is Executive Editor of IGN TV. You can follow him on Twitter at @EricIGN , IGN at ericgoldman-ign and Facebook at Facebook.com/TheEricGoldman