The 38-year-old actress appeared on Thursday's episode of Watch What Happens Live, where she recalled her scene with Ledger in the 1999 movie where the actor sings "Can't Take My Eyes Off of You."
Stiles and Ledger, who died at age 28 in 2008, played high school students Kat Stratford and Patrick Verona in 10 Things I Hate About You. Patrick (Ledger) famously sings to Kat (Stiles) from the bleachers in the film.
Stiles denied rumors that it was her idea for Ledger to sing "Can't Take My Eyes Off of You," but said the scene was a standout moment during filming.
"My favorite memory of that day? The whole day he was just phenomenal, I mean, running up and down those stadium steps. Nobody knew that he had this amazing voice, and he just went full out and I got to watch it," she said.
Stiles and 10 Things I Hate About You director Gil Junger previously discussed the scene in an interview with The Huffington Post in March.
"Heath surprised everyone with his commitment to that performance," Stiles said.
"Heath was wildly magnetic, but I got to say, just those looks that Julia Stiles gave, from 'Ugh, it's that [expletive]' to 'What's he doing?' to 'Who's he singing to? Me?' to 'Oh my God, I love that man.' I mean, she did it all," Junger added.
Stiles most recently starred in the film Hustlers, which opened in theaters in September. She also plays art curator Georgina Clios on the Sky Atlantic series Riviera.