Hawke: Movie like being in a band
Ethan collaborated with French actress Julie and director Richard on Before Sunrise in 1995 and then on sequel Before Sunset in 2004, and the Training Day star said it was like no time had passed when they came back together.
Speaking at the Los Angeles premiere of the movie, he revealed: "I get into the room with Rick and Julie and the characters start talking to each other, it's like being in a band, you get in a room and you know what instruments you play."
He added: "If you had asked me in 1994 if I was going to write and star in a trilogy and still be talking about it 18 years later I would have thought it was a ridiculous idea."
The 42-year-old said playing a role over so many years has prompted him to reflect on his own development over time.
"How could it not? Whenever you embark on something, it's our job to relate to these people and create something meaningful to us."
Walking the red carpet in a floor length red lace gown, Julie said her character Celine has evolved even more than Ethan's.
"I feel men have a tendency to stay close to the little boys they have always been and that's a beautiful quality, they believe in fairies. Women have to grow up and deal with their lives, especially if they want to achieve anything."
Before Midnight is released in UK cinemas on June 21.