When expanding a short story into a novel, the easiest thing to do (for me, at least) is to bloom out the supporting characters.
The minor players should offer more than contrasting bursts of color; in a novel their presence should tickle all the senses. Give them body, a flow, a scent, a cadence of thought as rich as the leads. They will add so much more to your book, once you add so much more to them.
Until tomorrow…
Chloe