The style of the narrative perspective is carefully crafted to allow for a fluid, compelling and immediate relation of events while also offering enough detachment in the latter sections of the novel to provide journalistic context for the case trial.
This flexible narrative point of view generates a meta-narrative, self-critical quality in the text that defines the book's fundamental character as a work of probing reflection as to the psychology of violence and the intelligibility of the social fabric in America.
The somewhat detached narrative point of view makes it possible for the narrative to describe the legal machinations of the case, which would not be possible with a narrative perspective that was attached more closely to Perry Smith or Dick Hickock, the killers.
Citing legal precedents and exploring the nature of an insanity defense argument in a murder trial, the narrative is able to deal with ideas far outside the purview of the central characters in the story. Thus, extended discussions of research into murder cases conducted by parties called upon to testify in the trial can be included in the book. Without such discussions, the narrative would inevitably hew toward an arc that not only resemble fiction (as In Cold Blood does in its published form), it would be indistinguishable from fiction.
The expansive narrative perspective then can be said to function as a means of distancing the form of Capote's book from that of fiction, making it journalistic and objective.
There is a notable elasticity to the narrative perspective that provides In Cold Blood with some of its thematic power, which derives in part from the narrative form Capote employs -- a hybrid formula that plays upon certain resonances that exist between the parallels drawn in the book.
"The 'nonfiction novel' derives much of its power from the tension between Hickock and Smith and from corollary dichotomies between reality and fantasy, masculine and feminine, plain and gaudy (eNotes).
The journalistic elements of In Cold Blood are used in such a way as to directly comment on the meaning of the narrative elements of the book.
For instance, when psychiatric research into murder and murderers is shared, an excerpt is quoted in Capote's text that offers a rather pointed intimation as to the psychology of Smith and Hickock.
"'Despite the violence in their lives, all of the men had ego-images of themselves as physically inferior, weak, and inadequate. [...] Guilt, depression and remorse were strikingly absent."
Generating a self-interpreting narrative structure, Capote is able to put the act of interpretation into his book and in doing so develops a theme relating to ideas of how meaning is made (and how meaning is ascribed to behavior, often post hoc, in the service of a sense of societal integrity and intelligibility).
The book raises questions as to how to understand deviance and violence and to what extent personal history shapes a person's ability to freely make choices. In structuring the book so that these questions are foregrounded, Capote's use of a detached and elastic narrative perspective can be seen as a well-considered means of creating a work of journalistic import and engaging story-telling, with both qualities working together to define the character of the work.