great film but book in the book Fowler is much more nasty. SPOILERS

I think it's just that the book gets more inside Fowler's head. Movies can't really convey people's inner thoughts the way books do. I wouldn't say Book Fowler is "nasty", either. Just jaded and world-weary, skeptical of Pyle's evangelical way of thinking and its consequences for the world, and of course worried about losing Phuong.

Pyle isn't such a great guy either, frankly. He was an imperialist who had no concern for how his actions affected others, whether individuals or countries. The combination of his childlike naivete, evangelical zeal, imperialist self-interest, and vast means at his disposal was immensely destructive to Vietnam. He was like a child with too much power in his hands, having no idea how much harm he was causing. Also, it's worth noting that Pyle isn't just an individual character; he was written to represent American interventionism itself. Pyle is Graham Greene's critique of American imperialism. One could just as easily write a book about a Pyle in Syria today as in Vietnam back then, going around someone else's country talking about "democracy" while giving bombs to fascists. Pyle is the timeless "ugly American".

Agree. In Anatomy of a Scene, Michael Caine admitted that he deliberately softened the character to make him more relatable. Really wish they wouldn't do that, but it's quite common with actors.