Tim Powers in The Guardian
"I look for a situation or historical character or place that looks likely to have elements that will make a good book," he says. "Of course I'm just guessing, but at this point you think 'that looks like a rich field'. And then I read extensively: biographies, journals, ideally contemporary travel guides, things like that, always looking for something that is too cool not to use."
I like the expression, "the audience doesn't know, but they know." When you get the details right, even though the audience doesn't know they're right, it somehow feels that the details are right.
One advantage of rooting his stories in the real is, he hopes, that readers will be more likely to suspend their disbelief. "It gives a lot of real-world lumber to support my crazy supernatural business. I'm always very aware of the risk that a reader will blink and say wait a minute this is all made up crap, isn't it?" he says. "But if I talk about carriages and shoe buckles and George III and commerce between London and Amsterdam, the reader will be a little more tilted towards thinking this is happening in the real world. If I wrote about the magical kingdom of Ding Dong and the lost prince and the dark lord, I would have ceded a whole lot of plausibility."