That said, Before Midnight is much more of a risk than Before Sunset. That film had "only" to re-establish a romance that the audience probably all wanted to see return. Here, we're another 9 years down the line and, unlike the preceding 9 year gap, Jesse and Céline have spent the entire time together. So making their extended romantic interlude interesting for the audience isn't a given. Simply making the film another enjoyably rambling conversation between two people that slowly edges them towards hooking up at the end wouldn't work, and the film's writers - who include its two stars - don't take this path.
To be fair, there is plenty of enjoyably rambling conversation [**], but it's used more sparingly, and positioned between scenes in which the conversation between Céline and Jesse is much more edgy than before. And the conversation doesn't stop at edgy, tipping over into anguish as the fractures in their relationship are first exposed and then explored. As such, it's not an easy watch like its predecessors, but in allowing the stresses and strains of long-term relationships (the demands of children, fading attraction) to be painted in realistically, it's much more grown-up. Satisfyingly so, ultimately.
As such, and much like that towering giant of successful threequels before it, Toy Story 3, Before Midnight shows other films how to do it. Not just a simple, crowd-pleasing retread of its predecessors, but one which retains their charms but finishes them off with a recognition of deeper waters beyond easy film screen romance. Whew - I'm totally glad they pulled it off.
[*] Not forgetting the brief appearance of Jesse and Céline in the also-excellent Waking Life.
[**] "Enjoyable", that is, if you've already bought into the modus operandi of the films of extended, playful and semi-philosophical banter between the leads.
In passing, I can't resist mentioning Jesse's outline of his planned new novel to his holiday friends. In just a few minutes, the film sketches out what could be a great little book (or, arguably, film), based around characters who're all intriguingly cognitively impaired. I'd read it already.