In the 2004 film, the second of Hawke's Before Sunrise trilogy, the actor's character Jesse reconnects with a French lover, played by Julie Delpy - and now his mum is revisiting an old romance.
He explains, "A couple of years ago... she got an email from a guy (that read): 'You may not remember me, but when I was, like, 17, we met once and I just have always wondered how you are.' She wrote back, 'Of course I remember you, David.'"
It turned out the old flame, a father of six, had recently divorced and he became pen pals with Hawke's mother.
He adds, "My mum kinda made up a reason why she had to go to London... and she said, 'I have this business trip in London, would you wanna meet me there?' So, after about 40 years they met in London and it was on. They got married.
"I thought, 'This would be a great sequel to Before Sunset,' and I'm thinking about this... and I ended up calling (director) Richard Linklater and saying, 'Y'know, this would be great... We wait another 25 years or something and we can do this think about how they (his and Delpy's characters) missed each other and then we'll have this really amazing thing when they're old and I could base it on my mum."
But Hawke didn't want to wait until he was old to make the third Before Sunset film, and his mother's romance inspired him to create the script for new movie Before Midnight.