It's quite rare that I'm inspired to write a novel; it's usually a character who acts as the kernel around which everything else grows. Short stories are more likely to start from a neat idea.

As to how plot arcs come about, they start out a bit like short stories, after which I write until I realize I've gone in the wrong direction and start over. I do a lot of rewriting -- I'm prolific, so it's a process that works for me. I've never been able to stand back and plan a novel; it's essential that I have my hands dug right into it.