“10 Things” Director Recalls Heath Ledger's 'Old Soul' amid Late Actor's 45th Birthday (Exclusive)

In a chat with PEOPLE celebrating 25 years since '10 Things I Hate About You,' Gil Junger says of Ledger, "I cast him basically before he even spoke"

Twenty-five years after his breakout role in 10 Things I Hate About You, the late Heath Ledger is being remembered fondly by the movie's director, Gil Junger.

Speaking with PEOPLE amid the teen comedy's milestone anniversary, Junger, 69, says that he "cast [Heath] basically before he even spoke."

"This guy was an old soul if you've ever met one," the filmmaker adds of Ledger, who would have celebrated his 45th birthday on April 4.

The actor was found dead on Jan. 22, 2008, in his apartment. A toxicology report revealed a fatal mix of prescription drugs in his system.

"It crushed me when he died, because I knew who he was. I knew he was not a drug guy," Junger says of Ledger. "People change, obviously, but when I knew him ... he wasn't a drug guy at all."

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Related: Remembering Heath Ledger's Life in Photos

Not long before the actor died, Junger called Ledger's agent Steve Alexander and told him he "finished this script that I basically wrote with [Heath] in mind" — which would be called 10 Things I Hate About Me and, at the time, "was going to be a film that explored and exploded the myths of suicide," though the theme of the still-in-the-works movie has since changed.

According to Junger, Alexander told him that Ledger "would love to work with [him] again," but had made the agent "promise that for the next two weeks until [Heath] completes filming, no phone calls, only unless it's an emergency," as he was finishing up production on The Dark Knight at the time.

Recalling Alexander's comments, Junger continues, " '[Heath is] having a problem turning the character off at night. And he's having tremendous trouble sleeping because he's so deep into this complex character. He asked me to just let him be. No calls, no nothing, because he was having so much trouble sleeping.' "

"Ten days later, he was dead. From drugs to help him relax, break anxiety and sleep," the filmmaker says. "It's so sad."

Related: Christopher Nolan Honors Heath Ledger During Golden Globes Director Speech as Robert Downey Jr. Tears Up

Junger — who, through The Breakthru Company, offers audition workshops and courses — tells PEOPLE that by the time he met Ledger for a 10 Things audition, he "had already read over 200 guys" for the role of bad boy Patrick Verona, while casting had "probably read close to 2,000."

"[Heath] walked in the room in one of these big casting sessions ... I swear on my parents' life, I thought, 'If this f---er can speak English, I'm going to cast him,' " Junger recalls. "He just had an energy that was undeniable."

So undeniable, in fact, he adds, "I liked Heath so much as a person that I would literally ride to and back from work with him just to spend more time with him."

Recalling the then-teenage Ledger's "bizarrely mature" nature, Junger says he told the young actor his "life [would] forever change" once 10 Things came out, and advised him not to "get lost in the bulls---. The friends that you have now should remain your true friends through your entire career."

"Any boy at 17 hearing that from a quasi-known director would be like, 'Oh my God, he thinks I'm going to be awesome. I can't wait. Holy s---. I'd better buy a house,' " Junger adds. "And you know what his response was? 'Yeah. Well, how about we worry about today? Because all I really want to do is be the best I can today.' "

10 Things I Hate About You is streaming on Hulu and Disney+.

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