"Just like the characters our relationship is deepening," says 42-year-old Hawke. "I mean, it gets more complex the more we know each other. We've now written three films together. That's an incredibly intimate thing to do."
The first film didn't carry the actors' names on the screenwriting credit. Originally, Linklater employed another of his former actors, Kim Krizan, to help him with the female perspective. Nonetheless, he searched for actors who could contribute to the dialogue.
"Nineteen years ago, when I was casting that first film, I was looking for the two most creative people I could find," says the 52-year-old. "I knew this whole film relied on those two people. There wasn't anything else. That script ended up being an outline with ideas sprinkled throughout that we built on."
"In the audition he asked me, 'Do you write?'," recalls Delpy. "I was like, 'Oh, I wrote a screenplay'. So he wanted that."
By then, Hawke had already written a novel, made films and directed music videos. He was the James Franco of his time. They were both in full-blown acting careers. Hawke had arrived with a bang starring in Dead Poets Society in 1989, then moved into heartthrob territory with Reality Bites.
Delpy, born in December 1969, is almost a year older than her co-star. She was the rising star of French cinema and had appeared in Europa Europa and Krzysztof Kieslowski's Colours trilogy.
The series that the films are most compared to is Francois Truffaut's films about Antoine Doinel.
"But those films are all so totally different," says the actor. "Each film is unique. These ones are almost turning into one long film. Which is very different."
It's funny to revisit the argument Jesse used to persuade Celine to spend the night with him in Vienna. "Jump ahead, 10, 20 years, and you're married. Only your marriage doesn't have that same energy it used to have. You start to blame your husband. You start to think about all those guys you've met in your life and what might've happened if you'd picked up with one of them. I'm one of those guys."
It's the plot to both sequels. The first inkling of a sequel came in 2001 with Linklater's rotoscoping animation Waking Life. Jesse and Celine are in bed analysing a conversation from Before Sunrise but, as with everything in that movie, it turns out this was a dream.
Before Sunset takes place nine years after Celine and Jesse agreed to meet again in Vienna.
Despite the lack of plot, Before Sunrise had left us with a cliffhanger ending: do the pair who refused to exchange numbers meet as they promise six months later? Romantics and cynics had something to argue over.
When environmentalist Celine shows up at married author Jesse's book reading in Paris it's soon clear they did not meet in Vienna. Roaming the streets, catching up, the walk has another ambiguous ending with Jesse in Celine's apartment threatening to miss his flight home. "We have to watch the films every time we do a new one," says the actress. "I mean, we haven't done that many, but on Sunset we had to watch this one, and on this one we had to watch both." The actors met up and watched a double bill in one night.
A further nine years down the line and Before Midnight casts Celine and Jesse as that frustrated married couple. There is an inevitability that they would get together, after all June 16 is "Bloomsday", the date that James Joyce first went on a date with his wife-to-be Nora Barnacle.
Fitting with their status, the formula of the films has been remodelled. Hawke says, "The two most difficult things about the third one are no ticking clock, and the fact that they've now known each other for so long, why would they be telling the other person?"
For the first time we see the couple interact with friends and their focus is on maintaining a relationship. Fans will not be disappointed. Once again, the strength is that the dialogue and events feel like we're eavesdropping on friends. It feels too real to be fiction. Hawke jests, "She says to me in this movie, 'You have sex the exact same way every time', you know, and that's something that my wife has said to me."
Linklater is full of praise for the naturalism of his collaborators, "It's a tribute to Ethan and Julie how well rehearsed and what good actors they are.
"It's the kind of acting they don't really give awards to because people accept it as real. It's kind of like the apes at the beginning of 2001 - they didn't win best costume because they thought they were real. People think we just turn on a camera and we catch these dialogues."
There's a rule that there has to be unanimous agreement on the script.
"We write each other's lines all the time," says Delpy. Ultimately, in the editing room Linklater has the final say.
"So many directors have this dictatorial attitude," states Hawke. "'It's my film'. His whole thing is he wants everybody in the movie to feel like it's our film."
Linklater is a director who is often credited with putting his home-town of Austin, Texas - where Hawke was also born - on the film-making map. So it's ironic that these films set far from Austin are the works he will be remembered for most fondly.
"We were thinking about setting this one in the US," says Linklater. "Maybe in a town like San Francisco. We have to think, where would she be able to get a job in her field that is fulfilling to her? And where would he maybe, as a writer/teacher, where would he be? So, you think of places that could work. We'd pick up with them on a Thursday - she's at her job, he's doing his thing, they'd meet in the evening - what life is for a lot of people, domestic. And then we were like, 'That's kind of depressing', but we thought about it for a while and the final film reflects some of those things but the idea was he's on this writer's retreat."
In the end they chose Messenia, Greece, where Linklater knew of a writer's retreat. Jesse and Celine are holidaying with family.
One of the great beauties of the series is that practically everyone can relate to the idea of the one that got away. At the end of Before Midnight, there is a dedication to Amy Lehrhaupt. Linklater met her in a toyshop in Philadelphia just after he'd made his directorial debut Slacker. They spent the night walking and talking and this became the inspiration for the series.
In reality, they swapped numbers, but after a few letters and phone conversations, they lost contact. It was back when Mark Zuckerberg was still in long shorts. Three years ago, Linklater discovered Lehrhaupt had died in a motorcycle accident on May 9, 1994. Just one month before Celine and Jesse met.
What: Before Midnight, the sequel to Before Sunrise (1995) and Before Sunset (2004) starring Ethan Hawke and Julie Delpy, and directed by Richard Linklater.
When: Opens in cinemas on Thursday.
- TimeOut / Independent