"We would just hang out with each other and, like, listen to music," recalled Gordon-Levitt, who is currently in production on five films and has been nominated for two Golden Globes since his appearance in the 1999 comedy.
"We spent all of our downtime together and loved it, [and] perhaps I got some fake IDs," Union -- who recently joined the panel of judges on America's Got Talent -- shared with a laugh. "It was just such a special, special time, and I could never recreate that."
Stiles was just 17 when she landed her first leading role in 10 Things I Hate About You, while her on-screen love interest, Heath Ledger, was just 19 as he played the handsome bad boy in his first major big screen appearance.
"Julia and I work together. We're fantastic together," Ledger said in a behind-the-scenes interview during the production of the beloved comedy. "She's beautiful, she's so sweet, when she smiles she lights up the camera."
Looking back at the time she spent with her late co-star -- who tragically died less than 10 years after the film came out, at the age of 28 -- she recalled what it was like to work alongside him.
"He was like this cool older bad boy that had just gotten off the plane from Australia… he had such a zest for life," Stiles shared.
The film, which was loosely inspired and adapted from the Shakespeare comedy The Taming of the Shrew, differed from a lot of high school rom coms of the day, focusing on a strong, driven female lead. It was this independent and non-conformist persona that really drew Stiles to the role.
"When I first read the script, I thought, 'Oh my God, this is so refreshing, this girl is so feisty,'" she explained. "I was kind of an angsty teenager at times and I felt like it was finally like somebody had captured that voice."
For Union, shooting 10 Things I Hate About You was such an incredible experience, she told ET that it sort of set an unreasonable expectation for what being on movie sets would be like for the rest of her career.
"We had a great, great time, I remember thinking all movie making was going to be like that," she recalled. "[But] I've never had an experience quite as special as 10 Things I Hate About You."
While Stiles -- who currently stars in the Ovation TV drama Riviera -- said she has a slew of fond memories from her time filming the project, she admitted that she doesn't really revisit the film often, or re-watch it when she sees in playing on TV.
"I immediately change the channel," she said. "Its like if you went looking through your high school yearbook. You might cringe a little bit."