The most important characters in an author's universe...?

Today I'm delighted to welcome John F Allen to my blog, author of The Best is Yet to Come. Not long ago, I read and enjoyed his Ivory Blaque novel, the God Killers, which if first in a fascinating series. The Best is Yet to Come contains one Ivory Blaque short story, so I wondered, how do authors combine writing series with strong, memorable characters, and then writing short stories... So, over to you John...

HOW I COMBINE WRITING A SERIES AND SHORT STORIES
I find that depending on the idea...