When a notion mentioned in passing in one work becomes the full story of another work by the same author. Most often a book trope.
It is related to Noodle Incident, which when explained, becomes a Resolved Noodle Incident. The latter could be considered a Sub-Trope of this one, additionally requiring the story to be framed and referred to in a specific manner.
- Nevada - Courtesy of Wikipedia: "The origin of the character is to be found in a Howard the Duck story that contained a "mandatory fight scene" between a Las Vegas chorus girl, an ostrich and a standing lamp. Neil Gaiman said he'd like to see that story. So when Gerber was asked to come with something original by Vertigo editor Karen Berger, he created Nevada."
- Terry Pratchett's The Amazing Maurice and His Educated Rodents started out as a passing reference in Reaper Man.
- Koom Valley is repeatedly alluded to when dwarfs and trolls come into conflict as being the battle where both sides ambushed each other. In Thud!, it turns out it was going to be a secret peace meeting, but when a fog fell someone yelled "Ambush!" and the battle that wasn't supposed to happen did, and served as a Remember the Alamo / excuse for warmongering since then.
- Star Wars Legends did this kind of all the time. Not only is a backstory provided for every character who appeared onscreen in the Star Wars movies (and even some of the Faceless Goons), but you also get stories that explain exactly what a "nerf herder" is.
- China Miéville's Bas-Lag Cycle is full of these. Among a great many other things, the protagonist of The Scar is mentioned in one line of Perdido Street Station as being Isaac's last girlfriend.
- Sauron was briefly alluded to in The Hobbit. Gandalf tells about how he got the key to the Lonely Mountain from Thorin's father, who had been captured and driven mad by him. However, he was called "the Necromancer", therefore making it difficult to realize who was being discussed.
- The main character of Mercedes Lackey's short story "Wide Wings" was originally a minor character in The Black Swan, her adaptation of Swan Lake, one of the potential brides that the Prince rejects in favor of Odette. According to Lackey, Honoria stood up and announced that she wanted her story told, and she didn't really care what Lackey wanted.
- The Silver Chair takes a brief moment to discuss an old legend as part of dinner entertainment, and says one day that story would have to be told in its entirety. That legend became The Horse and His Boy.
- Several of Ciaphas Cain's (HERO OF THE IMPERIUM!) exploits were referred to in passing, notably his participating in clearing out a Space Hulk alongside Space Marines. This is the subject of "The Emperor's Finest", where we also learn the origin of one of the women he lists as being terrified of (the others being an Inquisitor/his paramour and the other a daemon princess), a clingy planetary governor's daughter. "Cain's Last Stand" turns it Up to Eleven when the origin of a single quote from the beginning of a chapter is revealed ("Well that was unexpected" -Chaos Warmaster Varan the Undefeatable), and in a much darker context (Instead of the last words of a guy who Didn't See That Coming, it's the triumphant shriek of Varan revealing that in addition to a Compelling Voice, he also has bulletproof skin and talons. This then swings back the other way thanks to a hilariously one-sided fight and undignified death).
- Cherry Wilder wrote three novels set on the alien world of Torin, and also several short stories exploring the culture in more detail or providing resolutions for characters who only appeared briefly in the novels. For instance, in The Luck of Brin's Five the protagonist takes part in an air race where one of the other aircraft crashes and the pilot is killed; there is a short story in which the engineer who set up the race visits the pilot's family and we learn more about the pilot (and the engineer).
- Several short stories set in the Liaden Universe are examples:
- In the novel Carpe Diem, one of the main characters is driven through the city of Solcintra by a snarky cab driver who caught the imagination of fans and prompted many to write in asking for more about her. The novella Skyblaze gives the cab driver a name, Vertu dea'San, and tells what happened to her during after the climactic battle that occurred in Solcintra a few novels later.
- In the novel Scout's Progress, one of the chapter heading quotes is a message home by a pilot, one of the hero's ancestors, giving a brief account of a stopover at space station where he was overcharged for emergency repairs. "The Space at Tinsori Light" is the story of that pilot, and reveals that there was something much more sinister going on at the space station than the brief account would indicate.
- Douglas Adams's Starship Titanic started out as a passing reference in one of the The Hitchhiker's Guide to the Galaxy books.
- The Legend of Zelda: Ocarina of Time is an expansion of the prologue detailed in A Link to the Past's intro cutscene. There is one deviation from the prologue, though: in Ocarina of Time, Link lives.
- An optional scene in Final Fantasy VII reveals that Jenova used her own cells to infect the Cetra tribes with a virus, nearly wiping them out. By the time of Final Fantasy VII: Advent Children, this virus, now called "Geostigma", has been brought back by Sephiroth.