There's a good little article by Nicholas Shakespeare on Graham Greene's voice and technique here.
Without wishing to pick too many recondite nits, Shakespeare makes two mistakes. First, in the list of influences it misses out Eric Ambler who pretty much created Greene's world, or at least presented it to the bookish public. The Third Man is essentially a (brilliant) reworking of The Mask of Dimitrios. Greene once said that Ambler was the writer to whom all spy novelists are indebted*.
Second, Greene is eminently parodiable. You need a skeletal style, a sense of physical inferiority, loneliness and an eye for decay.
It was autumn in the late
All the other tourists had left the resort. The Place Des Etrangers was empty and several of the bars had taken the opportunity to close up for the winter. I remained.
And that's without mentioning Catholics, brothels, diplomats or South America. To prove my point, in May 1980 The Spectator set a competition for parodies of Graham Greene. It was won by Graham's brother, Hugh. Graham's sister, Elisabeth Dennys, came third. Both were writing under pseudonyms. Graham's own entry failed to place, but eight years later he used it as the opening of his last novel, The Captain and the Enemy.
Here it is:
I am a man approaching middle-age, but the only birthday I can distinguish among all the others was my twelfth. It was on that damp misty day in October that I met the Captain for the first time. I remember the wetness of the gravel in the school quad and the blown leaves which made the cloisters by the chapel slippery as I ran to escape from my enemies between one class and the next. I slithered and came to a halt and my pursuers went whistling away, for there in the middle of the quad stood our formidable headmaster talking to a tall man in a bowler hat who carried his walking stick over his shoulder at the slope like a rifle. I had no idea of course who he was or that he had won me the previous night at backgammon from my father.
Oh, and Greene's best book was Travels With My Aunt.
*I can't find the exact quotation right now. I may fill it in later.