Research is like good make-up.
It should make you look better, without drawing attention to itself.
As a writer, you have to do far more research than you'll every use in your book.
But once you've done it, you're tempted to show it off.
Resist the temptation.
There's nothing quite as off-putting as having your characters indulge in a long conversation about the history of Rome, just because you looked it up and you're damned if you'll lose it.
In contrast, visiting a place once is not enough to set a story in it.
You have to know it intimately.
You must have pounded its streets, you must know it's history and customs, you must have sat with locals and listened to their anecdotes.
If you don't know which way your characters must walk, and whether they must take a bus, train or taxi, to get where they want to go, you're not ready to use it as a setting.
You'll be so busy avoiding what you don't know, that your writing will be stilted.
Not only that, but a novel is far more than the bare bones of plot.
The magic of a story is in the way it can transport you to another time and place.
And to do that, the texture of that time and place must be enough.
The devil is in the detail, and their must be enough specific details to give your readers the sense of a place - not just the way it looks, but the way it smells and sounds and the way the sun falls at a particular time of day.