Richard Linklater's film, which follows on from 1994's Before Sunrise and 2004's Before Sunset, recently wrapped a secretive shoot in Greece.
Hawke and Julie Delpy will reprise their roles as star-crossed lovers Jesse and Celine, who met on a train in Before Sunrise and spent only 24 hours together, before being reunited 10 years later in Before Sunset.
"The expectations have risen so much [with] these movies," Hawke told Hitfix. "When we did the first sequel, nobody really cared... there weren't any expectations."
He went on to explain that while he, Delpy and Linklater, all of whom co-wrote the script for Before Midnight, are very conscious of their responsibility to fans of the films, their main priority was telling an emotionally honest story.
"Obviously we wanna make the fans happy, you know, and give them what they want. And at the same time, you can't make everybody happy. You just can't do it," he said.
"So we have to try to just have the same attack and just kinda try to be ruthlessly honest and really tell the story about where we think these people are in their lives right now."
Pressed for more details, Hawke would only say: "It's all just as mysterious as it's always been. The point about life, you never know what's gonna happen."
Although Delpy initially denied that the film would shoot this summer, Hawke confirmed in August that production had begun in Greece.
Watch a trailer for Before Sunrise below: