Guest were part of the cast for Regretting You, the movie adaptation of Colleen Hoover’s novel. Guest played Ellie, the childhood best friend of Clara, portrayed by McKenna Grace. It was Guest's very first acting role—technically, Guest didn’t even audition. One day, McKenna and Mason Thames (who plays Miller) happened to run into Guest and immediately decided Guest had to be in the movie. They said Guest looked exactly like how Ellie was described in the book and insisted no one else would fit the role better.

Since Guest was still under 18 and your parents couldn’t travel with Guest, McKenna—who was just a year older—was technically Guest on-set guardian. Guest ended up spending a lot of time with both her and Mason, on screen and off.

In the story, McKenna and Mason’s characters were the ones dating, while Ellie (Guest character) often played the third wheel. But fans who read the book developed a popular headcanon that the three characters were secretly in a poly relationship and just kept it hidden. McKenna and Mason loved teasing that idea while filming. During kissing scenes, they’d sometimes squish Guest between them just to mess with the crew—and Guest

Rumors about McKenna and Mason dating off-screen exploded after McKenna posted a TikTok of the two of them kissing. What no one knew was that she had another video in her drafts where Guest were sandwiched between them and they both kissed Guest on the cheek. She never posted it —but it ended up becoming one of her favorites.

She took a screenshot of the moment and made it her lock screen, Mason did the same thing with his lock screen, and Guests was a picture of the three of you during a scene where Guest were squished between Mason and McKenna during a slow dancing scene.