Union, 44, and Ledger played Chastity and Patrick in the 1999 comedy 10 Things I Hate About You. Nearly a decade later, the Aussie star died at age 28 from an accidental drug overdose on January 22, 2008.
"God, the first time I met him, he had done a movie in Australia and nobody had really heard of him — he was just this mysterious 19-year-old," Union told WWHL host Andy Cohen. "He told me and everyone else who was over 21 to meet him at the top of the hotel at this bar, and we walk in, and he’s just got this wild hair. Again, he’s 19, but he’s got a dad drink."
From then on, the cast was inseparable. The Being Mary Jane actress revealed that they ate dinner together every night and would always split the bill. "He was such a man, such a gentleman, and his girlfriend at the time was like 38," she said. "He was great, warm, funny. We had a lot of fun. I thought all movies were like that."
Union added: "The whole cast was like a dream. But he was a doll."
The Gil Junger–directed flick also starred Joseph Gordon-Levitt, Larisa Oleynik and Julia Stiles, Ledger's onscreen love interest. Stiles, 35, opened up about her own experience with the Oscar winner during an interview with Us Weekly in October 2014.
"He was so nice. He was such a force," she said while attending the Orphaned Starfish Gala in NYC at the time. "He was, even at that age, a very, very powerful, lovely human being."
Watch Union on WWHL in the video above.
Can't get enough of Us? Sign up now for the Us Weekly newsletter packed with the latest celeb news, hot pics and more!