Capote is, as the title indicates, as much (or more) a movie about the writer as it is the crime he wrote about. But the most interesting part of the movie is not the character of Truman Capote, no matter how well-performed, but the book that he writes. And even then, the movie treats it as a side note, even as it's being constantly praised by almost every character.
What I find fascinating about the book, as told by the movie, was that it's based in large part on Capote's own actions--he hires lawyers to keep the subjects alive, cajoles them, plays them against each other--but he is, if I remember correctly, mostly absent from the actual book. This is entirely in keeping with the "true crime" journalism that the piece purports to be, but of course it's as much novel as non-fiction--Capote relied on his memory for quotes and recall of interviews, and he retold large parts of the story, including the internal dialog of several participants.
In this way, Capote points out, the author begins to think of the people in his book as fictional characters--he's increasingly distraught and upset as they refuse to give his work a quick, snappy ending. Eventually he avoids the calls and telegrams coming from the killers, hoping to distance himself from them, just as he distances himself from the events of In Cold Blood within its pages.
Without In Cold Blood, I wonder, would we have missed out on the entire genre of novelized true crime that has now become typical (The Devil in the White City, perhaps, or Under the Banner of Heaven, which bears no small resemblance to its predecessor)? Probably not: a central conceit of journalism for some time now has been the invisible narrator--the reporter is assumed to be irrelevant to the story being reported, by both the readers and the reporter themselves. In some cases this is true, but often it is not. And the device of dramatization allows those latter cases to be hidden behind a thick layer of prose. Who knows how much the observer is actually entangled with the observed--and who cares, when it's such a good yarn?
Which is a bit of an ironic message, coming as it does from a dramatic retelling of the real Truman Capote's character flaws.