Credit: Netflix

Voicemails for Isabelle, the Netflix movie that throws Zoey Deutch and Nick Robinson back to their rom-com roots, hinges on the idea that when people experience excruciating loss and grief, love is not far down the line. But what viewers might not know is that the story also has a real-life inspiration that they must know about.

In an exclusive conversation with PEOPLE, director Leah McKendrick, who also stars in the movie in a minor role, revealed that the idea for Voicemails for Isabelle actually dawned upon her at the most improbable spot.

According to McKendrick, she had written Voicemails for Isabelle seven years ago. The rom-com follows Jill, a young, budding baker, coping with the death of her sister, Isabelle, by leaving her voicemails about her chaotic daily life. Unbeknownst to her, Isabelle’s older number is reassigned to an elusive realtor, Wes, in Austin, Texas, who, instead of deleting the voicemails, eventually falls in love with Jill without even having seen her, and subsequently, seeks her out.

Although the central theme of sisterhood came naturally to the writer-director, the voicemails reportedly had to do with a comedy set she had witnessed in which her roommate participated. The set had a skit about how her father would send multiple voicemails describing every minute update of his day, which was followed by the next comedian taking the stage and responding, “It’s so nice that your dad calls you. My dad hasn’t called me in three years.”

As the overwhelming reaction from the crowd poured in, the comedian went on to reveal, “He’s dead.” “I was the only one that laughed. And then it really got the wheels turning, and I thought to myself, it’s so funny, this idea of a girl who keeps waiting for her dad to call her back.”

McKendrick thought to herself about the situation, picturing her sister, which made her believe that she would probably wait for the same too. “And then I thought, no, if my sister died, I would just keep calling her,” she said. The idea was born right there, but after her sister actually relocated to New York for college, the director really started sending her several voicemails about her everyday life.

“I would just let it all hang out,” she added. “And what a horror story it could be if somebody were to ever hear my most unfiltered self. But you would know that if somebody fell in love with that unfiltered self, it would be real.”

Voicemails for Isabelle essentially serves as a “love letter” to McKendrick’s sister. “I think if you’ve experienced true love through family, you don’t settle when you’re searching for it in romantic love because you know what it feels like,” she explains. “And some people don’t know what that feels like to really have true love. And I’m very deeply grateful for the fact that I knew from pretty much [when] my sister was born when I was four.”