Development Hell: Part of the reason minor characters are so fleshed out is that the story is the result of years of failed drafts. For example Panacea and Glory Girl were originally planned to be the protagonists and already had a story arc plotted out.
Wildbow has said that Panacea and Glory Girl were the original protagonists of the story when he was first developing it, under the original title of Guts and Glory. Both the Travelers and Faultline's Crew were also the protagonists of their own stories before they were folded into Worm as well. At one point, Lisa's code name would have been Deduce and she would have been a fellow student at Taylor's school. These earlier snippets are now available on Wildbow's blog.
Quite a few interludes exist only because of reader donations. It's easy to imagine them never coming to pass had things fallen short of the quota.
Word of God: Wildbow occasionally gives a few info dumps about the world and various topics in the Wormverse. Past attacks by the Endbringers, clarifications on powers and unseen characters, etc. One compilation can be found here, and another here.
Despite Worm being finished, Wildbow continues to answer questions about the story on some forums. Mostly, these are clarifications about disputed issues of Fanon.
I started writing when I was in middle school. I was the unholy trifecta of geek, nerd and Cloud Cuckoolander, a disabled kid, and I was miserable. For me, writing started out as a way to vent, like some people hit punching bags or scream or go running. I was inspired by Buffy, so I started out writing modern supernatural stuff and it was basically me writing the most messed up stuff I could.
Writing by the Seat of Your Pants: Wildbow makes a point of this as one way of making the writing process more interesting for himself. Virtually every chapter was written in the day or two immediately before publishing, often finishing shortly before the midnight deadline, and he had set himself a minimum-length of 6000 words. Over the course of the story's run, he missed the target time of an update only twice — both by mere minutes, and both times due to technical issues.