I did The Quiet American for year 11 Literature and ended up not only enjoying the novel but was able to write on it to a reasonable standard as well - I ended up duxing the year level for English and Literature. In my view, if you're reading it for the first time, read it for fun, don't worry about themes, messages and quotes, you'll have time for that later. Once you've finished reading, try and imagine that you're each of the main characters. Think of what motivates each character, how they go about their life and what happens to each character by the end.
Just a heads-up, there will be spoilers in the following paragraphs.
The film is decent. My only issue with it is that Pyle seems too confident. For me, Pyle is supposed to be completely innocent and have absolutely no idea that the destruction he wreaks is intolerable and inhumane. Although, Michael Caine is a pretty good Fowler, he fits my image of the oldie quite nicely.
If you're looking for a few scenes which pretty much sum up the entire novel, think of the Watchtower scene where Pyle and Fowler are stranded and chat to each other revealing each of their world views. My favourite line from the text is in that scene, Fowler yells, "Who the hell asked you to save my life?" to a Pyle who's expecting the older man's praise. Also think of the scene where Fowler sees the small dead boy curled up in a ditch, this image recurs throughout the novel. For me, it becomes a symbol of the carnage and heartbreak which war brings to Vietnam. Like Fowler, Greene intends for us to be utterly dispirited when we think of how much damage is being caused to people who are so innocent.
Finally, on a thematic level, it's probably central to the novel that you understand that Fowler is an unhappy man. His misanthropy towards life filters his narration. You'll notice that most of what you read is in past tense signifying that it's Fowler's recollections you're reading. Not only that, Fowler is an embodiment of the idea of escapism: he's come to Vietnam to escape his marriage at home and views Phuong as freedom to which he clings to make him feel happy. So when Fowler sees Pyle dancing with Phuong and comments that the younger man was a shocking dancer, think of how much of that is really true. Do you think Pyle really is that bad? Maybe. But certainly Fowler's disgust and perhaps desperation that a younger, more attractive man is dancing with his only chance for freedom may cause his narration to be skewed.
P.S. You might like to know that all of the letters Fowler receives from his wife in England are actually real. They are the letters verbatim that Graham Greene's wife sent to him. I found this out while doing some reading about the text during year 11. Talk about authenticity...