What motivates me? Hmm... that's hard to answer. In a way I can't say that I am 'motivated' to write. I 'have' to write. I am 'driven' to write. When I have a story burning in me I can't do anything but write. It posessed me every minute of the day until it is spent and then, more likely than not there will be another one burning to get out.
I find myself working out dialogue on the train, picturing scenes while I am supposed to be working. Walking, talking, eating, breathing like my characters as I work out what they would do and wouldn't do, what they look like, who they are as people.
What kind of things spark off an idea? Anything. A mannequin in a shop window, something someone says in conversation, attending a funeral, being bored in court and looking around at my fellow lawyers... anything and everything.
I'm probably a complete freak but hey, that's the way I like it.