Romance Ensues

This trope occurs when two characters, usually with a healthy dose of UST between them, are forced into a situation where they are alone with each other, away from prying eyes and the infamous Moment Killer, for an extended period of time. During this time, these two characters are allowed to interact with each other freely, without any unpleasant interruptions.

This usually results in the characters getting to know each other better, and either developing feelings for one another, or having those feelings strengthened (And perhaps even acted upon) if they existed previously. This usually happens later in a series, but it can happen earlier, and indeed be the progenitor of the UST between the two characters.

This trope can take the form of a romantic getaway to a cabin in the mountains for a weekend, two people who are on the run from the law for some reason living together for a period of time to keep up the facade that they are a married couple, or simply two friends being locked in a shed together at school for some time by accident (or by a group of friends trying to get them to hook up). This seems to occur more often in works with at least a dash of Romantic Comedy in the plot, if not taking up the entirety of the plot. It may also occur when a marriage of convenience or a friends-with-benefits arrangement crosses the line into being something more.

Examples of Romance Ensues include:

Anime and Manga

The plot of the second half of Samurai X: Trust and Betrayal: Kenshin and Tomoe, on the run from the local law enforcement in Kyoto, are forced to move to Otsu and live in a small homestead as husband and wife to avoid suspicion. The characters were acquainted with each other before, and hints were made at some attraction between them. During this time living together, with little interaction from the outside world, their feelings for each other develop into love, they end up marrying, and consummating their love for each other.
This is the plot of at least one episode of the Lovely Complex anime. Otani and Koizumi have obviously had a thing for each other for a while, but they just can't seem to show it. Their friends notice this, and get fed up with it. So after a game of basketball, their friends hatch a plan to lock the two of them in the equipment room for a few hours, not letting them out until they've had sufficient time to interact and (hopefully) finally hook up. Unfortunately, it takes a little longer for them to actually get together, but this whole event helps them to get closer to each other.
Black Lagoon: In the episode with the crew of Neo-Nazi Nitwits, Rock and Revy are forced to dive down and explore the ruins of an old WWII submarine. In the confines of this derelict craft, Rock, who didn't know much about Revy up to this point but had been fascinated by her, is able to get to know her a little better, share some life experiences, and advance to the stage of friends before they're picked up by the rest of their crew.
Spice and Wolf: really only a small part of each of the episodes deals with either of the main characters, Horo or Lawrence, interacting with anyone besides each other. Most of the time they are traveling from place to place with each other, or sharing a living space with each other. With all this time to themselves, it makes sense that their attraction to each other will grow. (ok, not the best example, but running a little thin at the moment)
Each of the girls of Hayate the Combat Butler gets at least one of these (many of them have had two or three), whether it involves an actual date in which the others have agreed not to interfere, or simply their own arc treatment. The title character's Oblivious to Love mindset uses these to only strengthen the UST between them however.

Film

In at least the first Austin Powers movie, Austin and his new partner Vanessa rent out a hotel room in Las Vegas together, according to Vanessa "to keep up the appearance that we are a married couple on vacation" while they spy on the henchmen of Dr. Evil. At first Vanessa is put off by Austin's behavior, but after they work together for a while, go out for a night on the town, have a little drinking party, and share a few serious moments, Vanessa ends up falling in love with Austin, resulting in their marriage by the end of the movie.

Literature

In the The Druid of Shannara, Morgan Leah and Quickening are separated from the rest of their group when she falls in a river and Morgan jumps in after her. (She wasn't really in any danger, being an elemental, but he's fallen in love with her.) When they get out of the river some distance away from the group, they have to wait for their comrades to catch up. During that time, they converse and Morgan discovers Quickening has similar feelings for him. A short-lived romance ensues courtesy of her upcoming death.

Video Games

In Metal Gear Solid 3: Snake Eater, after loads of UST between main character Snake and Femme Fatale / Sidekick Eva throughout the game, Volgin is killed, the Shagohod destroyed, and the Boss is defeated by her former student, allowing Snake and Eva to return to their homes, but not before making a stop at a nice little cabin in the mountains where Snake does what he's been thinking about for days by laying Eva down and making sweet love to her on a bearskin rug in front of a roaring fire. Far away from the enemy armies, and the prying eyes of their respective governments, at a place just for them, Snake and Eva finally allow their romance to flourish. If only it didn't have to be so short...

Web Comics

After a long time building up UST between Joyce and Walky they find themselves suddenly with no duties toward their agency thanks to the government shutting down S.E.M.M.E. and find themselves lodged at an inn together. Release of long unresolved sexual tension as they realize that this time there is nothing to get in their way and that they are finally both ready for this step ensues, with consequences for the structural integrity for the building.

Live Action TV

iCarly's episode IOMG subverts this trope as Carly locks Sam in a room with a guy Carly THINKS Sam likes due to an Ipear app Freddie's group makes saying Sam's "In love." Sam is not impressed. The show then plays it straight afterward when Sam is moping on some steps outside leading Freddie to try to cheer Sam up; believing they're alone, Sam kisses Freddie after he tells her that everyone is afraid of telling someone how they feel.