When it comes to creating ideas for a book or characters, every writer has different ways to find inspiration. Movies, TV shows, music, news; there are so many things out there that can spark an idea and help create a whole new world. For me, most of my inspiration comes from historical research, music and photo shoots of the models that I use as my 'muses'.
The other day, I was researching one of my characters, Buer, who happens to be an actual historical demon, and came across some information on him that I hadn't seen in my previous research. One word stuck out for me and that one word created an entire story in my head. I never tended to write a story about Buer and his wife, Renata (the King and Queen of the Demon Hell Realm, Gehenna), but reading that word had made me change my mind.
Sometimes a story will start and there will be characters involved that don't need a back story. Sometimes those characters aren't ready to tell it to you yet. (I have this problem with Berith. The man is very tight lipped about his background). Then when you least expect it, that voice that usually stayed so quiet just starts talking and that character that you once thought as a 'minor character' transforms into something that you never expected.
Never let yourself stress over the fact that a voice in your head is silent. They'll tell you their story when they are ready and not before hand. Just like your readers when they read your stories, a little mystery is good for the soul. Trust me when I say, the wait is worth it.