For my comic: I know most authors do this, but having a few, very in-depth characters fleshed out before they start writing? (I'm talking like habits and favorites/dislikes, detailed backstories) I'm more of a "shotgun" character builder - there's tons of them appearing with very little development, even trope-ish in fact, but of course I intend for the plot to form them into more well-rounded characters, unless they're meant to be in the bg. With a growing cast, this does make it tough to figure out which characters to develop at certain times, like being overwhelmed with picking a candy at the cash register XD.
Also, for writing a to-be AF novel, getting sidetracked in research, or too deep. I've read a bunch of Arthurian legend literature, and Beowulf, Holy Grail analyses, and itching to rewatch Game of Thrones all in order to figure out the "atmosphere" I want to write in-there's no indication that I'll actually use anything from these influences, just killing time. I'm also having trouble creating an interesting protag (backstory-wise, really) but it's three months young, so I have time to figure that out. Definitely on my own journey atm.