When fans last saw Elizabeth Gillies star on screen, she played sassy student Jade on Nickelodeon’s Victorious.

Now, she’s left high school behind to go full badass as Denis Leary‘s daughter Gigi in Sex&Drugs&Rock&Roll – a show about an aging but ambitious rocker created by the Rescue Me alum.

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And she’s bringing her soulful vocals along for the ride.

“The best part of the show, really – there’s great music, there’s great comedy, the cast is brilliant – but it’s about a dysfunctional family and just the most extreme characters coming together and arguing and fighting and coexisting, and there’s this musical element that makes it all a little bit crazier,” Gillies told PEOPLE at the ATX Television Festival over the weekend. “It’s a lot of fun. But there’s drama, too, so it covers all bases.”

Read on for what the 21-year-old had to say about being an old soul, jamming with the cast and her bond with “sister” Ariana Grande.

When you were reading the script, what made you say, "I have to be a part of this"?

It was such an interesting project, and it was so different from everything I’d been reading. I always feel old to play my age just because I have an old soul, and when I read this character, she talked like me and she said things I would say, and then there was this music element. It was really the perfect role for me, and I almost felt like it wasn’t real. I had to get it no matter what, so I fought really hard, and I’m very thankful that I got it. It’s weird when you see yourself written down, almost, and this is what this was like for me.

Denis is so brilliant, and our minds work very similarly, so I really wanted to work with him. I think it really sells that I’m his daughter.

What is it like working with such amazing actors like Denis and John Corbett?

It’s wonderful. It’s really, really cool. Because Denis like to work on the fly a lot, he likes to imrpov a lot, we all come into it with the same kind of exhilaration, the same excitement to go in there and do some things completely spontaneously because we go off script so much. It was really cool, it’s a huge honor to get to work with those guys. I’m the youngest by like 10, 20 years on the show, so I was a little intimidated, but we all coexist on the same ground and we play like a real family, so it feels totally natural.

What about the music?

Denis wrote or co-wrote all of the songs, so it was all original music, and he wanted me to sing live. That was very important, so we made sure of that. I recorded some backups, God forbid I have a cold or something like that – we never really had to use them. We did live takes for everything, and it just makes it feel much more believable. It keeps you in the moment, like you’re watching a rock concert as opposed to something pre-recorded. So I was happy he wanted me to do that, and the songs are great. He’s brilliant. He directed and wrote and produced and wrote all the songs. It’s extremely admirable.

Were you able to collaborate on the sound of your songs?

Not with the writing, but with the arrangements of the songs, the keys and kind of the production, we totally collaborated. We were in the studio for weeks before we even started filming because the music is such an integral part of the show – it’s so important. We wanted to make sure we got that just right, and I think we did. It was super collaborative, which never really happens. It’s been really collaborative with Denis and I, I’m really thankful that he’s allowed me to have so much say in things, but mostly we follow his lead. He’s the genius behind the whole thing.

It seems like there are a lot of shows about the music industry now: Empire, Power, Nashville.

It’s exciting. It’s exciting for me to see music integrate with TV, because I love them both so much. I came from musical theater, so I was always pairing the two, and then I was lucky enough to be on Victorious, the Nickelodeon show I was on, and that had a music element and an acting element and comedy. But this – to get to do drama and comedy and improv and to sing – I’m so lucky. I hit the jackpot.

What’s it like to go from Victorious to a show like this, which is decidedly more mature?

I’d say so! It’s fun. It’s very liberating. It’s cool. Even when I was on Nickelodeon, I talked and had the sense of humor of someone much older than me, so it’s fun for me to kind of be myself more on this. Not that I wasn’t then – I was younger! I did Victorious when I was younger, so I think it’s just a natural progression. It makes sense. But it is cool. There were no stepping stones. Usually you do a couple things that lead up to something more mature, like an ABC Family or a CW [show]. I just went right to FX, which was cool! I’m happy to do it. I didn’t think I’d get to.

When you’re acting as a child or teen versus a more adult role, do you approach it differently?

Nickelodeon is bigger, it’s louder, it’s much more structured. It was a sitcom, so the laughs had to go where the laughs had to go and the jokes had to be played a certain way, and you do multiple takes and it has to be the same way every time. This one, there would be takes where Denis would tell me to just make up all my lines for the entire scene, and we used most of that. So it was really extreme opposites. It couldn’t have been more different.

I love that you were in 13 with Ariana Grande, and then you were on Victorious.

We can’t get away from each other! And we did an animated show together, we did voiceovers on this show for Nickelodeon together. We really can’t escape each other.

Are you still close? You’re both incredibly busy.

I think she’s busier than I am! She’s in Europe now, but we’re totally close. She’s like blood to me, she’s like my sister. So unless we have a massive falling out, nothing will be happening there. [Laughs] We lived together, I’ve known her since what feels like the beginning of time, but really it’s been seven years since all this started.

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What women on TV do you look up to?

I love an old-school kind of lady. I love Jessica Lange. … I think Amy Schumer‘s really cool, I think she’s killing it. Kristen Wiig. A funny woman is really becoming a thing today, and I’m happy that it is because women are so funny, and it’s cool to see that, to have the same comedy as men sometimes.

There’s rougher kind of humor on the show, but the women on the show – Elaine Hendrix, who plays Ava, and I – we totally hold our own with the guys. It’s fun to see men and women go back and forth in that way.

You know who was my favorite growing up? Megan Mullally on Will and Grace. That for me was just the pinnacle for comedy. She was so hysterical, and she had that fantastic diva essence to her and that voice. I just love that! I love a really good bitch, too. Not someone who knocks your books over in the hallway, but someone who wears a fur and walks slowly and says something brilliant and leaves. I’m a firm believer in the old-school, ’40s strong woman, leading lady. That’s my favorite.