Literary sub-genre: Novel or play retells a classic from the perspective of a secondary character or characters. The new story tracks the the original but shifts some of its action offstage. The two versions intertwine, each now commenting on the other.
Examples:
Rosencrantz and Guildenstern Are Dead (Tom Stoppard).
James (Percival Everett).
@JamesGleick "Sycorax", a play by Susan Gayle Todd.
In part a prequel to "The Tempest", it is the story of Caliban's mother, how Caliban came to be born on the island, and Caliban's relationship with Miranda.
@JamesGleick I enjoyed reading Wicked, the retelling of The Wizard of Oz from the wicked witch's perspective, exploring the politics and economics of Oz in great detail...
@JamesGleick Pretty certain either I or my parents have one besides that one, but I can't remember what it is, and my own bookshelves aren't accessible currently. On the other hand, I might just be thinking of that one, or maybe of something I'm trying to write or a game I'm trying to make (eg. a Kozure Ookami game entirely from Daigoro's perspective...he's not exactly a minor character, but he's a toddler for most of the series).
@JamesGleick Grendel by John Gardner, and "The Yellow Wallpaper" short story by Charlotte Perkins Gilman is often cited as a retelling of Jane Eyre from Bertha's perspective.