Andrew Milito’s review published on Letterboxd:
It's not long into Before Midnight before we realize that something's off here. It's Ethan Hawke's Jesse and Julie Delpy's Céline another nine years later, this time vacationing in Greece, but they're different now. The idealistic nature of the two completely unrelated souls that were seemingly meant to cross paths in Richard Linklater's Before Sunrise is gone, and taking its place is are two reflective, troubled souls. This kind of somber storytelling was introduced in Before Sunset, sandwiched in between Sunrise and Midnight, but the latter pushes it even further. All seems well at first, with Jesse and Céline engaging in the same kind of intellectual conversations, and ultra long takes of course (one take involving a long car ride is particularly impressive), Linklater has seemingly perfected, but problems slowly but surely arise as this 18 year relationship begins to take its toll on our protagonists. It results in a tense standoff between the two characters that makes them, and us as viewers too, question if this relationship was truly meant to be. It's absolutely heartbreaking and truly bittersweet, and the film as a whole is most likely Linklater's most mature outing to date.
The Greek setting seems to mirror this relationship rather well. It's an undeniably beautiful sight to behold, even despite its age. Perhaps it was more beautiful in the past, but like all things, age has taken its toll, and it's not quite the same as it once was. Maybe this is just all coincidence, but don't put it past someone as intelligent and philosophical as Richard Linklater.
(I'm sorry if this review is kind of awkward, because I truly have no idea how to review this movie. Maybe I'll come back and edit it sometime, but I'm just completely at loss for words right now.)