I've heard "write what you know" many times. But what do I really know about Vampires and Demons? About Time Travelers? Honestly, I only know what I've read from other authors.
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Ghost? Well there's more freedom there because there are so many types of Ghost. I watched Ghost Hunters one day they had a marathon. This lovely paranormal researcher and a sexy ghost started taking shape in my head. From that came Ghostly Passions (releasing June 1, 2011 from MuseitHot Publishinhg)
And then I took on Death. While most books whoever is Death is male. But one author, W. Somerset Maugham, wrote a short story "Appointmentin Samarra" in which Death was a beautiful woman. It stuck with me. So I start thinking after rereading Piers Anthony's Incarnations of Immortality series. Oh I didn't want to try to touch that one but the idea of the different incarnations having a physical presence? Oh yeah, I liked that idea. Which led to my current WIP! I'm still not sure of the title but the two in the running are My Lady Death and Death and the Detective. In which I also mix in my love of Romantic Suspense novels. I just need to finish it.
So if you want to write - then write. The ideas are all around let your brain mix the ideas around and you'll be surprised what comes.