It Happened One Night

Peter Warne (Clark Gable) is a hard-bitten reporter. He loses his job, but finds a ticket back in when he stumbles on to a runaway heiress, Ellie Andrews (Claudette Colbert). He meets her on a cross-country night bus, and threatens to blow her cover unless she gives him the exclusive story about her escape. They hate each other at first; when they realize that they'll have to share a room, they invent the "Wall of Jericho," a blanket between their two beds to keep them apart. But they eventually fall in love...

This Romantic Comedy from 1934 was directed by Frank Capra. It was the first film to win all five major Oscars in the same year (Best Picture, Best Director, Best Actor, Best Actress & Best Screenplay - a feat only repeated twice since with One Flew Over the Cuckoos Nest and Silence of the Lambs), made Clark Gable a star, and let the world know that Frank Capra was a great director. Its title was the solution to the very first rebus on Concentration.

It's also the first Screwball Comedy — it started a whole genre of comedy films in which a straight man-style character is chased, harassed, and eventually romantically captured by a Cloudcuckoolander (or related trope).

Tropes:
 * All Girls Want Bad Boys: Peter is kind of a jerk to Ellie in some scenes. She still falls for him anyway.
 * Blackmail: Peter wants the scoop on Ellie's flight to her husband; he threatens to call her father unless she gives it to him.
 * Also, Oscar Shapely wants to go 50/50 on a reward he sees in the newspaper of $5,000.
 * Clark Kenting: Peter and Ellie fool the detectives by pretending to be a low-class married couple. Somehow it works.
 * Crowd Song: Though the bus's rendition of "The Flying Trapeze" is believable.
 * Deadpan Snarker: Ellie to Peter during his hitchhiking attempts.
 * Fairytale Wedding Dress
 * Fan Service: Clark Gable was seen without an undershirt in this film. There is an urban legend that this hurt undershirt sales.
 * Have a Gay Old Time: "Dyke's Auto Camp" might elicit a few snickers today.
 * Money Dear Boy: Both leads were only in this film because of contractual obligation with the studio.
 * Non Indicative Name: It takes place over several nights and days.
 * Parental Marriage Veto: The heiress has run away because her father is trying to annul her marriage. Of course, this happens before she meets Clark Gable.
 * Road Movie
 * Road Trip Romance: The Trope Maker
 * Romantic Comedy
 * Seinfeldian Conversation: Ellie and Peter spend quite a bit of time discussing hitchhiking techniques and what does or does not constitute a piggy-back ride.
 * Gable has one of his own, where he complains that women "don't know how to dunk" (donuts into coffee). Nike used that scene for a shoe ad in the 80s.
 * Show Some Leg: The Trope Maker, and so old that many people don't even realize this is where it comes from.
 * Shirtless Scene: Gable's scene caused a media sensation.
 * Slap Slap Kiss
 * Sleep Cute: Early in the film, Ellie falls asleep on Peter's shoulder, while riding a bus.
 * This Is My Side: Having fun dancing around the Hays Code with that bedsheet. Has been homaged/used dozens of times by everything from The Sure Thing to Neon Genesis Evangelion to A Very Brady Sequel.
 * Eva, sure. But it's not surprising that they used it in The Sure Thing because that movie is a remake of It Happened One Night.
 * Worst News Judgment Ever: Ellie's love life makes the top story on the front page of every single New York newspaper. Evidently, not much else was going on in New York City in 1934.
 * Considering that it was in the middle of the Great Depression, chances are that any more upbeat news would've been more than welcome.