The 20-year-old actress recently called ET from Syracuse, New York, where she is in town filming her new movie, Big Time Adolescence, with Saturday Night Live star Pete Davidson and rapper Machine Gun Kelly.
"It's a coming-of-age story and it's hilarious," Sweeney said of the Jason Orley-directed film. "Pete adds a whole nother level of comedy to the script. My character, Holly, is a girl who's super quirky -- she has pink hair, she plays Zeke's girlfriend. But that's pretty much all I can say right now…"
"It's definitely different. I've never quite done anything like this," she continued. "I mean, [playing] Emaline in Everything Sucks! was lighthearted and that was a lot of fun, but this is definitely a nice, little break from everything else I've been doing."
Zeke is played by Davidson, who recently got engaged to Ariana Grande. Sweeney told ET that the singer has been visiting the set and hanging out with the cast and crew during breaks from filming.
"[Pete] is a blast; it's non-stop laughter [on set], and Ariana's here too, so it's just a big family, pretty much," Sweeney explained. "Everyone just goofs off and has fun."
"This is the first time I've ever met Ariana, and she's super nice," she added. "She's been hanging out with us all week and it's been a good crowd."
When the cameras aren't rolling, Sweeney says they've been spending most of their time checking out local restaurants in Syracuse, like Dinosaur Bar-B-Que and Funk 'n Waffles. "We've been eating a lot of food," she said, laughing. "We like food, we like to eat!"
While fans won't see Sweeney in Big Time Adolescence for quite a while (a release date has yet to be announced), they certainly won't have to wait long to catch her on the big screen. She recently landed a leading role in a horror film, portraying a possessed teen named Alice in Along Came the Devil, out in select theaters and VOD Aug. 10.
"The whole demonic possession part of it, I wanted to dive into it as much as I could," Sweeney recalled. "When I was preparing for the role, I actually went to a vocal coach where I learned different languages and how to contort my voice, and to a movement coach where I learned how to contort my body in different ways. I really wanted to make sure that the different levels of possession you saw felt clear and different between each and every one."
"I wasn't actually personally scared at all," Sweeney added of taking on such an intense role. "I think my mom was a little worried. She was like, 'We need to somehow figure out how to cleanse the hotel room afterwards.' I think she got more superstitious about it than I did, but I thought it was cool and you don't get that many opportunities to play [that type of character]."
It's seemingly all about the character for Sweeney, who also stars as Alice, Camille's (Amy Adams) roommate in a mental hospital, in HBO's Sharp Objects. The character received little more than a mention in the Gillian Flynn novel the series is based on, leaving it up to Sweeney to really bring Alice to life (and tragic death by drain cleaner).
"I really wanted to make sure that Alice had so many different layers to her… so I created this book that was like a timeline/scrapbook, memory journal of her entire life, and I kind of just added all these layers to a character like Alice that I was able to portray, that I could just jump into her shoes," Sweeney said, adding that she creates journals for every character she plays. For Alice, the actress filled her notebooks after watching videos of girls struggling with depression and self-harm.
"One hundred percent [I wanted] to make sure I portrayed her for everyone, because teenage girls struggle with a lot and go through a lot, and growing up is the hardest thing ever," she expressed, noting her intense disappointment when nothing "magical" happened on her 18th birthday. "There's that scene where she asks Camille, Amy Adams' character, if life gets better, and Camille basically says, 'No, it doesn't, and you kind of just have to live in it,' and I think when I was filming that scene, it hit me really hard, because I think that is true."
"[I realized], 'Wow, I've been waiting for my life to get better in whatever circumstance or thoughts I was having, it's really just what you make of it yourself, and you kind of just have to go with it,'" Sweeney added of the scene with Adams, whom she called her "cool older sister, mom, role model." "She's an incredible actor. She's an incredible businesswoman. She's a producer. She's also a mother at the same time, and being able to see someone being able to carry and handle that is really inspiring."
Arguably Sweeney's strongest role this past year, however, was portraying the recurring character of Eden Blaine in season two of Hulu's The Handmaid's Tale, based on the Margaret Atwood novel of the same name. Fans who watched the series couldn't stop talking about Eden's heartbreaking death by suicide during the series' penultimate "Postpartum" episode -- and for good reason. The scene set the story up for the finale, and showed us what the women of Gilead may do in season three to protect their children from the horrible expectations of the dystopian society.
"It was a combination of crying and screaming at Eden for being the stupidest teenage girl ever," Sweeney said, when asked how she reacted to learning of her character's fate. "I was just so mad at her! Ugh. But that's what young love is."
"I knew that the character was going to have an ending at the end of season two, but I wasn't exactly sure what her way out was going to be," she continued. "At first it was just going to be like, 'Oh, Eden and Isaac [Rohan Mead] ran away together, bye.' But the writers and producers all wanted something impactful for my character because everybody started falling in love with Eden on set."
She added, "It was such an incredible experience and they wanted to give me something that would really be meaningful for the series and for the audience as well. So, they took it that way."
Next up, Sweeney will be starring alongside Zendaya, Maude Apatow and Storm Reid in Euphoria, a series following a group of high school students as they navigate drugs, sex and identity.
"It's looking good. It's an amazing project and I think it would be crazy if it didn't get picked up, so fingers crossed," she told ET, just days before HBO announced its series order on the project -- and added Drake as an executive producer.
Unsurprisingly, Sweeney has a couple more projects in the works, one of which she'll probably be filming while she celebrates her 21st birthday on Sept. 12. But she's not complaining.
"That weekend will be, like, the Emmys and all that, so I'll be busy doing whatever I have to do. But I haven't thought about [plans]… I completely forgot!" she said, proving she's come a long way from that girl who once cried after placing all her hopes and dreams on her 18th birthday.