The Devil's in the Details

In the comments on our last exciting episode, accio_aqualung asked:

So pretend you've spent so much time on something that you've got gobs and gobs of backstory and little trivial details, like the MC is  terminally left handed or her brother has to organize his pens in a very specific way or their uncle won the very first US Open. These things have nothing to do with the plot but add humanizing quirks to the characters that would make them so much more interesting in real life. How do you...