With the movie celebrating its 20th anniversary this month and many paying tribute on what would have been Ledger's 40th birthday this week, Union is the latest to speak out about working with the late actor.
Ledger tragically passed away at the young age of 28 after an accidental prescription drug overdose in 2008.
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"Me & Heath. When life was simple and no one cared we were a bunch a young actors having the time of our lives up in Tacoma/Seattle one summer," she captioned a photo of the two of them.
"Every time I saw Heath after 10 Things I Hate About You was the same," she continued. "All love, all smiles, all hugs, all grace. He will never be forgotten." Union then thanked costar Larisa Oleynik (aka Bianca Stratford) for sending her the photo and "reminding me of the all those amazing memories from that summer of fun!"
"We were in this hotel in Tacoma, Washington, I think we were coming back from rehearsals because he came late," Keegan began. "At that time there was no internet, I didn't really look, I think I probably had seen a headshot. I was getting on the elevator and right as the doors were closing, one of those classic moments, and the door just stops."
"This guy gets on and he's got a Didgeridoo," he continued. "I could just remember I was in character too and I was like, 'This f--king guy. Really? Long hair, Didgeridoo, Australian accent.' And that's the memory of meeting him."
Keegan then got serious and said, "He was absolutely such an epic part of this project...We love Heath."