I would read the book first and then see the film version:
http://imdb.com/title/tt0061809/
There's a certain irony of Robert Blake portraying Perry Smith. The nature of sociopathy (now pretty much dropped for "pyschotic.") The new film "Zodiac" gets into that same area. It always ticks me off when film criptics bemoan the subject matter of a serial killer, but "In Cold Blood" wasn't exactly in the genre. They had it in them, but their didn't go on to act on it. The crux of the matter was the supposed hidden money in the farmhouse which was never found. Okay, now I've talked myself into reading it again and it is in my library.
I think see the filmed version first and then "Capote" will reveal a lot more about this American tragedy and Capote's meticulous investigative reporting.
After all, it's really an in-depth reporting of the details of the story developed into a novelized form. The book was ahead of its time is some respects but there have been many others. "Burr," or "Julian" by Gore Vidal come to mind. Capote always touted that he had invented something new but it was actually still history, even if recent history, and involved heavy interpretation of characters and events. I think he did manage to get it spot-on. I don't believe I've ever read a description of the scenario of a murder scene as gripping. I met Capote several times at his Palm Springs home and he was fascinating to converse with, though a big inebriated (okay, I had had a couple of cocktails myself). I didn't perceive him as a very empathetic person and felt his attachment to Perry Smith had strong sexual tension involved. Capote, I think, was the consummate observer.