Sidenote Full Story
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.
- 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.
- The Star Wars Legends do this kind of all the time. Not only is a backstory provided for every character who appeared onscreen in the movies (and even some of the Faceless Goons), but you also get stories that explain exactly what a "nerf herder" is.
- For an example: Remember that brief blink-and-you'll-miss-it glimpse of the Millenium Falcon in Revenge of the Sith? Well, there's an entire book out about why it's there.
- 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.
- The main character of Mercedes Lackey's short story "Wide Wings" was originally a minor character in her adaptation 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 plaetary 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 Famous 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).