Michelle Gagnon raises the question of whether it's really necessary to visit places before setting fiction there. After all, to travel as much as Graham Greene did requires rather a lot of money and emotional detachment.
An example Gagnon gives is that Martin Cruz Smith only spent about a week in the Soviet Union prior to writing his excellent novel Gorky Park.
Ultimately Gagnon's question can only be answered on a case-by-case basis. Sometimes all that's really needed for story purposes is enough scenic detail that the reader can suspend disbelief – details often best acquired through a combination of library research, distant memories, and pure confabulation.
Rebecca Solnit, River of Shadows -- “There are things that are hard to photograph: guerilla warfare, the end of an era, the meaning of a place. And there are things it is nearly impossible to photograph: the subtle workings of the human heart, the wandering paths desire and fury take, the bonds of love and blood that tie people together, the decisions that tear them apart, the way that the most unprepossessing landscape can become home and thus speak of stories, traditions, gods that strangers cannot decipher from the rocks and streams.”
These things aren't easy to observe while traveling either but, with luck and effort, one can get at them somehow by reading and writing and redrafting and remembering/ transforming/ figuring out...