For starters, let's take the premise of the film. Best friends Celeste and Jesse have vowed years ago to be together forever. At the beginning of the film though, the couple have been separated for half a year and divorce is a procedural formality. Yet for all that's worth, they might as well be the happiest married couple alive with their effortless chemistry and intimacy together. Opposites attract: he takes it so easy it makes her laugh, while she supports the loveable and kooky other half. What's that about the divorce? They still hang out together and crack each other up. They still live in the same house (though he's relegated to the guest house).
It becomes the complete reversal of the Hollywood romcom where two random 20 or 30-something stars with no chemistry whatsoever are cast in a film where you can't really figure out what they see in each other, why they're getting together, and why the film really wants to push them together. Here for most of the film, we have a couple we're told are splitting apart yet you can't really figure what they don't see eye to eye on, why they're supposed to be moving apart, and why the film really wants to split them apart.
Urged by close friends who have their best interests at heart, Celeste and Jesse date other people. Again, it's a series of dates gone wrong that we've seen before in bog standard romcoms, just that they're here precisely because the formula needs to be exposed for what it is and subverted. Ditto for the gay best friend who dishes saucy romantic advice, played here by Elijah Wood, whose deliberately failed punchlines deflates the stereotype.
As a genre parody, Celeste and Jesse gets its goals done though like the modern romcom genre it's parodying, you have to wonder why they even made that, even though the characters are endearing and the jokes are amiable. Thankfully though, the film is far more than just that — it belongs solely to Rashida Jones's screenplay, which manages to spin in the background an engaging yarn about the pitfalls of being a modern career woman.