As the book progress, we see Garrigan changed by Amin, whose character is as overwhelming as it is in the film. Where the book differs from the film is in the area of Garrigan's culpability. In the film of the same name, you are left in less doubt of Garrigan's general innocence. In the book, you get the sense that Garrigan is more aware of Amin's wrong doings at an earlier stage, and so might be more culpable in the activities that Amin got up to.
The issue of Garrigan's culpability is the one thing that irritates me about the book. You learn earlier that he knows of Amin's activities early on, so why doesn't he do something about it (or just walk away if he can't do anything)? The book talks about fear (or reprisals from Amin) and the fact that he's Amin's thrall, but I found that less than satisfying (or believable), and is a reason why I rave about this book.