Centineo and Purser spoke with ET's Courtney Tezeno at the premiere about the movie's life-changing moment for its leading lady.
"It was my first on-screen kiss," Purser revealed. "Not my first real kiss thankfully but, yeah that was horrifying. I was not prepared for that at all but I think it really added to the authenticity."
Shannon Purser Says Her First On-Screen Kiss Was With Noah Centineo! (Exclusive)
And yes, she was nervous to be kissing Centineo, explaining that director Ian Samuels kept the two apart until the big moment. "I couldn't see him in his tux and the moment was so honest and nervewracking the way when you're a kid falling in love for the first time."
Centineo confirmed that the scene will leave everyone swooning, saying, "It was so much fun to film. The scene was great, super cute, gotta love it, you’re gonna love it I promise."
He added, on a scale of one to ten, "I’d give it an 11, that was an 11."
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"He's so charming like I mean he really is exactly what you'd imagine him to be just handsome and sweet and easy to get along with so it doesn't surprise me at all really," Purser said of Centineo.
Purser is, of course, famous for her role as Barb, the doomed best friend on Stranger Things, even earning an Emmy nomination for the part. But now, she's ready to step out of the sidekick role.
"It's the stuff I've dreamed about as a little kid and I'm finally here and I got very lucky with the cast," she said of her star turn. "Everyone is so easy to get along with and just supportive and encouraging, so I'm really over the moon and I hope everyone loves the movie as much as I do."
The 21-year-old is also thrilled that a role like Sierra came her way. "I think the perk is I'm a part of...a network, a streaming service that is so open to new creatives and new ideas which really means a lot to me," Purser said. "I don't know if I would have necessarily would have been a lead in a big budget Hollywood movie. I think this movie will hopefully change that."
Centineo added that the story will touch viewers. "I think people are really going to relate to this and there’s really a deeper message. There’s different layers of morals in this that I think everyone can get behind. I hope it sparks a certain amount of reflection in the viewers."
Sierra Burgess Is A Loser premieres globally Sept. 7 on Netflix.