A character who was previously thought to be straight suddenly reveals otherwise, to the surprise of other characters and likely also to the audience.
There are a couple ways this can be played. A character may be introduced with no mention of their sexuality one way or another, until they're revealed to be gay. Other times, a character who was never established as anything but straight suddenly is shown to be gay, even if previous episodes depicted them as straight. This can be because the author changed his mind about a character, is appealing to the LGBT Fanbase (or other kinds of fans, or is trying to incorporate some LGBT representation in a show that may not have much, especially in the case of a Long Runner (which may lead to the character being a gay Token Minority). While it may be jarring for viewers and the characters, and in some cases come across as a retcon, it's Truth in Television for many gay people who previously thought they were straight and has romances with the opposite gender.
Equally important is the reaction of other characters. Some may be surprised, but quick to accept the character's new sexuality; others may be in disbelief, which can lead to a lesson about assumptions. There's bound to be a character of the opposite gender who's disappointed that this character isn't romantically available; often, shooting down a straight romance is how a character comes out.
Can be implemented as a quick gag, in which case the characters may immediately become some variation of a gay stereotype (Camp Gay and Leatherman being the most popular), regardless of their previous character traits. A character may also try to invoke this to call others homophobic for doing something wrong to them, despite those characters never knowing about their sexuality.
If this is done via Word of God instead of within a work, it's Word of Gay.
Subtrope of The Reveal. Often done with a Straight Gay. Can be forced by introducing Situational Sexuality. Relationship Reveal is a chance to out a character with more buildup. For the race version, see Suddenly Ethnicity. Compare to Late Coming Out, where a characters waits years (decades) before finally coming out, and Adaptational Sexuality, where a character's sexuality is changed in an adaptation. Contrast Transparent Closet where it's obvious to everybody that the character is gay even before they come out.
