Alternate Character Interpretation: In the novel at least, Phuong is deliberately left open for this trope. We learn virtually nothing about her true feelings. Fowler and Pyle are also candidates for this, since the story is told from Fowler's point of view.
Crazy Awesome: Pyle, at least in the book, where he's portrayed more of just a misguided jingoist. At one point he takes a boat up to visit Fowler in North Vietnam without any apparent recognition that he's putting himself in danger. He also evokes this when he acts to save Fowler after the latter breaks his leg.
Harsher in Hindsight: Pyle's bumbling naivety seems like a commentary on America's ultimately ineffectual handling of The Vietnam War... until you realize that the book and the first film were both written well before the war actually started. Very prescient of Greene's part.
Moment of Awesome: Pyle's determined rescue of Fowler after the latter breaks his leg in a fall, especially when he has to carry him through the thick paddy waters and refuses to abandon him when Fowler insists on it.
Moral Event Horizon: Pyle endorses a terrorist strike against civilians as part of his attempt to set up a new political force in Vietnam.
Nightmare Fuel: The aforementioned terrorist strike kills several women and children. In particular, the haunting image of a mother tending to the remains of her baby is what prompts Fowler to abandon his neutral position. Pyle's reaction is no better, dismissing the blood on his shoes as something to be cleaned off before he sees the Minister.
What Do You Mean, It's Not Symbolic?: Phuong symbolizes Vietnam. Lampshaded by Pyle: "Let's just look at Phuong. There's beauty. Daughter of a professor. Taxi dancer. Mistress of an older European man. Well that pretty well describes the whole country, doesn't it?"
Further extended, Phuong represents the developing world as a whole, Fowler represents the aging—nay, dying—old European powers, and Pyle represents the Americans who, in replacing the Europeans, do not realize exactly how unsavory their new position requires them to be.
Given that the second film adaptation came out in 2002, it's hard not to see it as a commentary on the US's then-ongoing preparations for the ostensibly high-minded invasions of Iraq and Afghanistan.
Ironically the film was test-screened the day before 9/11, so this connection is purely coincidental. It was shelved for a year for fears it would be seen as unpatriotic.