I’m not sure if this is the right thing to do, and I won’t know until I actually start working on the draft, but one thing I wanted to do was to have storylines for each of my characters. I think that will give them all the opportunity to grow and keep them three-dimensional.
Not all of the scenes related to my supporting casts’ storylines necessarily need to be shown on the page, but I at least should know what they’re up to and what their personal struggles are. Of course, I’ll also have to seed in the minimum amount of detail for the readers to be able to follow each of the stories for when they do re-emerge onto the page.
I suspect that will be a tricky balance, but it is something I’ll work on after I have all of the important plot events down and start trying to combine them into scenes.