Many films are derived from novels and, in the normal way, it is unhelpful to compare the movie with the book, for the obvious reason that they are distinct art-forms, constrained by different technical limitations. However, this one really does have to be understood in the context of the book which engendered it.
Capote's book is a factual account of a multiple murder in a small Kansas town. Two young drifters plan a robbery which misfires and ends in violence. The book traces the course of the patient investigation which eventually brings the killers to justice. Because the book is a species of journalism, uncompromisingly anchored in fact, the film cannot help but follow suit, with the added burden that it must faithfully represent on the screen real persons, places and events.
The mean lives of Dick Hickock and Perry Smith are documented in stark monochrome. Panavision is used to powerful effect to show the wide, flat spaces of Kansas. Quincy Jones's atmospheric jazz score adds salt to the bleak images. The austerity of blue collar life in the Mid West of the 1950's is splendidly evoked as our two delinquents move through a rolling montage of Travelodges and diners, launderettes and interstates.
This is a film of straight lines. The flat, relentless landscape of Kansas generates horizons that are ruler-straight. Roads stretch into the distance without the hint of a curve. Slat blinds cast harsh bars of light across room interiors. The penitentiary scene is a symphony of geometric lines. Hickock and Smith have had their characters forged by incarceration, and we see that their 'outside' lives are, in a real sense, another form of imprisonment. The lines which enclose them denote the hopelessness of their existence.
The starkness is reinforced by neat, economical editing. The throwing of a light switch in the Hickock farmhouse carries us to the Clutter home, where a light is being turned on, and the words 'any crowded street' whisk us into just such a street. A cigarette butt is discarded, and its ugly cylinder becomes an electromagnet searching for the murder weapon. The Clutters' cleaner realises that a radio has been stolen, and we see the radio playing at Dick's bedside.
Once under arrest, Dick makes a powerful speech about tattoos. The detectives are trying to provoke him by sneering at his 'body art' and he points out that we all carry tattoos of some kind. Our dress, speech and attitudes mark us indelibly and fix us in our time and place.
Herb Clutter and his family lead a spartan home life. The farmhouse is spare and unadorned, but its order and solidity make a sharp contrast with the chaos and squalor of the rented rooms where Dick and Perry hole up. Dick 'hangs paper' (passes dud cheques) in respectable Kansas stores, amassing clothes and electrical goods on a spree which exploits the trust between vendor and consumer and uses it as a weapon - Dick and Perry's revenge upon 'decent' America.
Once the arrests have been made and a trial scheduled, the film switches to a voice-over narration. No doubt this was done in order to shorten the custody passage (this is extensive in the book, but does not lend itself readily to film treatment), but it jars. Up to this point, Hickock and Smith have told their story through action. Narration is second-best.