Bearing down on it now, this business of sub-genres and sub-sub-genres and the ethos and eros of romantic comedy, with the logical query: If Getting to Know You movies believe that true love is only achieved over the course of some serious time (in When Harry Met Sally, it takes more than a decade, and in Groundhog Day, a near-eternity), what are we to make of a rom-com that claims you can have it overnight?
Love Overnight movies, a small but hardy cadre of romantic comedies in which the main action takes place within one extremely compressed and compacted time period - usually a linear span of around a day and a night or so - are founded on the belief that love, whether at first sight or a later second's consideration, has no need of racking up considerable psychic or physical mileage. In these movies, The Real Thing can be sighted, experienced, and validated in record time.
The uber-Love Overnight pic, obviously, is Richard Linklater (and co-screenwriter Kim Krizan)'s Before Sunrise (1995),
in which Ethan Hawke and Julie Delpy do their meet-lose-get scenario in roughly 18 hours. That they blow their scheduled post-movie reunion, and don't hook up until about 16 years later in the sequel Before Sunset (filmed 9 years later, 2004)
is moot: that they do get together for good, Ethan impulsively abandoning a wife and child in the bargain, is proof that What Happened One Night was indeed a happening thing.
It Happened One Night (1934), by the way, covers a longer time span than its title promises, but it's an LO movie in spirit. These stories posit that what happens in Vegas does not stay in Vegas, but lives on - or, to cite the grandaddy of such "single at lunch, married by breakfast" tales, what happens in the forest near Athens changes you forever, even if you were only literally turned into a donkey for a few hours. Shakespeare's magical romp of an ensemble rom-com makes the case that one Midsummer Night's Dream (a couple of nights, actually) can hitch a few pairs of lovers, and better them, for good.
In this essential regard, Love Overnight (LO) romantic comedies are the diametric opposite of Getting to Know You (GTKY) movies, yet kissing cousins to the Only One paradigm - rom-coms that believe if you can find your fated soul mate, you'll be good to go. In the LO universe, if an attraction develops, is tested and endures over the course of one fated night, that's all its faith-and-hope-prone protagonists need in order to make a solid commitment.
2008's unjustly overlooked all-in-one-night rom-com Nick and Norah's Infinite Playlist - the answer to your future "What was Kat Dennings in before 2 Broke Girls?" trivial pursuit question - delivered the full, passionate goods in this neck of the woods: as in Sunrise, the meet-lose-get between Kat and Michael Cera goes down in one linear fell swoop, and it seems to be for keeps. It's a credit to screenwriter Lorene Scafaria that we buy into this idea... which may be why there aren't dozens of these LO rom-coms extant. Your average romantic comedy, even if it spans weeks, can be credibility-challenging enough.
The short list of hybrid Love Overnight classics over the years is topped by the horror/rom-com Shaun of the Dead (2004). Here the protagonists have met and broken up before the 24-hour story proper gets launched, but one night of bludgeoning, axing, and shooting flesh-hungry zombies together is all it takes to make its two lovers like one, again. 2007's teen/bromantic rom-com hybrid Superbad similarly has a prologue to zip through before its neverending party night brings both boy-and-girl, and boy-buddies together before dawn.
Another seminal LO movie is John Hughes' teen rom-com Sixteen Candles (1984), coupling Molly Ringwald and Justin Henry over the course of a pivotal birthday's non-celebration. Like the Shakespearian original, Woody Allen's minor Midsummer Night's Sex Comedy (1982) is a nocturnal roundelay of hook-ups. As every genre must have its nadir, we'll dutifully note that the execrable Valentine's Day (2010) follows form, its soul-killing superficial scenarios taking place in 24-hour holiday time.
A memorable variant is 1954's Roman Holiday, in which Gregory Peck and Audrey Hepburn spend a day and night together, a reporter and a princess on a platonic (but tacitly romantic) spree in Rome: no happily ever after for these two, but it's a time they'll remember all their lives. One knowing anti-LO classic, Scorsese's black comedy/rom-com After Hours, is like an urban nightmare version of Bringing Up Baby with no Katherine Hepburn in sight (the series of "madcap" women hapless Griffin Dunne meets are crazy in a chillingly unscrewball way). The movie suggests you not only can't get laid in a New York night, you're liable to get killed and/or turned into a plaster statue for trying to.
Living Rom-Com commenter Judith Duncan astutely noted that the GTKY movie lacks the high-gear energy of shorter-fused rom-coms. LO pics, by their very "zero to 60 in no time" nature, have well-tuned motors (commenter Cantara Christopher's suggestion - Vincent Minnelli's The Clock, a rom-com/dramedy - is a quintessential example). And if you ever needed evidence that rom-com lovers are optimists, the Love Overnight sub-genre, with its embrace of Be Here Now encounters and Instant Love Everlasting, clearly provides it.
So what other LO classics am I leaving out?