This wasn’t something I came up with; I deferred to the original player regarding these kinds of details. I added some details here or there and made some suggestions (and so did other players), but ultimately what became “canon” was up to the NPCs original player.
This approach honestly worked much better than I was expecting. In the original campaign, the criminal PCs all had a kind of loose background, with varying amount of detail as it came up in play. In the new campaign, with those PCs serving as NOCs being investigated, we were able to flesh them out from an entirely different angle, and the players were heavily invested given that they had made these characters.
It was a pretty unique situation, and one I don’t know if we’ll ever really get to repeat, but it really drove home how possible it is to give players a lot of ownership over NPCs and how it can enhance a game.
This is interesting and shares some similarities with the solo DW game I'm running for my wife that I discussed a little above, though in reverse.
We're constantly discussing various character ideas, and she was mulling several possibilities for our new game. One was the Svirfneblin Ranger-Psion she is playing; another the Tiefling Immolator that is now an NPC in that game. She wondered rhetorically what the relationship might be between these two very different characters should they ever cross paths, the former a meek slave whose impulse is often to blend into the background (via a psionic ability to cloud her foes' ability to perceive her a la invisibility bur for all the senses) and the latter a commanding, confident presence burning with the inner fires of the Hells themselves.
As we sketched out the setting together in the first session, taking turns on adding details and locations, I introduced a Tiefling kingdom, offering the possibility that such introduced the possibility of my wife's Tiefling PC character concept appearing in the fiction in some fashion. I introduced an unnamed female Tiefling in framing the initial play scene, but even as that scene played out we weren't sure if this Tiefling was my wife's imagined Tiefling.
In subsequent sessions, as the Svirfneblin PC has been dispatched on a fetch mission by her new owner, a powerful Tiefling ambassador and military officer, we decided it made sense for the hitherto unresolved character of one of his bodyguards to crystallize into this Immolator character, and we sketched her out as a Hireling mechanically, even though she is technically the PC's overseer in the fiction. The next several sessions should allow us to develop this NPC's character collaboratively, and there's always the possibility she becomes a full-fledged PC down the road, either in a later game or as a kind of secondary PC similar to what
@pemerton has described for his Traveller game.
EDIT ADDENDUM: As a specific illustration of how the NPC's character is taking shape and how this adds to the overall shared fiction:
When the Ranger-Psion PC's animal companion, a cave dog with the vicious tag and Instinct to savage its prey, killed a cave rat, I asked my wife to illustrate this, keeping the tags in mind. She described the hound tearing the large rat apart and devouring its heart. I took my opportunity and had the Immolator NPC comment, "Huh. I thought only Tieflings consume the hearts of their foes." So that's a thing now.