It’s easy to be focused on your main characters and to put your creative energy as a writer into just those characters you adore – the Good Guy and the Bad Guy, mainly – and leave until last (or leave out altogether) the small guys and gals that provide background support.
I’ve read a lot of books over the years and have seen lots of variation. I have seen books where the reader got the full life history of every single character, none of them important to the plot and none of them having anything to do with any of the others, until the plane crash or motorway multiple collision – some big accident, anyway – at the end of the book (the one you were shown a glimpse of at the beginning of the book). Basically, those books seem to be an exercise in writing characters, not plots, and I’m afraid they don’t do much for me.
I’ve also seen my fair share of Main Character in all his/her glory, right down to zits and all their foibles – completely and totally surrounded by 2D nothing characters that might have been made of cardboard.
There is life in between. It doesn’t have to be a giant pain and it doesn’t have to be ignored, either.
You can flesh out a character with a word or two. Flashes of human feeling or an expressive action will do it. Very simple things can do it.