1. Paul Newman was originally considered for the part of Hickock, but the relatively unknown Scott Wilson was chosen instead to enhance the film's realism. It was a good choice because Wilson actually looks and acts a lot like Newman. As much as I'd have loved to see Newman in the part, his celebrity-status would have diminished the raw chemistry Wilson shares with Robert Blake (who plays his partner in crime, Perry Smith).
2. I wonder how the film would have played had it not been directed by Richard Brooks. Brooks was mainly famous for directing adaptations of literature, like The Brothers Karamazov, Cat on a Hot Tin Roof, and Elmer Gantry. Despite its mature subject matter for the time, this is actually a pretty tame film, sticking extremely close to Capote's novel. I doubt Brooks felt he had the right to make it any other way considering that the book had only been released a year before. Brooks was a great director in my mind (he also made the wonderful, often overlooked western The Professionals) but he seemed to be someone who literally made things by the book. I'm not saying that's a bad thing here, especially considering the greatness of the source material, but I couldn't help but feel the movie felt slightly pre-programmed during its final thirty minutes...
3. ...which leads to the major flaw of the movie, namely its anti-capital punishment agenda. The agenda itself is not a problem, but bringing Paul Stewart in as a reporter who clearly thinks that individually these men deserve to live (he says that together they formed a third person, who committed the murders) definitely is. For a film that often feels European and naturalistic, this Ayn Rand-style preaching is extremely detrimental to the powerfully objective lens through which Brooks' paints most of the movie.
4. I mentioned the film seemed European, which may be the best part of it. Because we don't see the actually killing until the end, the first two hours of the film play like a long road trip with a couple of loners who don't have much to talk about but money and big dreams. It's actually pretty awesome, but the effect keeps getting dashed when the film gets procedural and keeps turning back to the police investigation (like lots of procedurals, it's not all that interesting). But those long scenes of Smith and Hickock driving down south and then back up again are some of the highlight moments of the New American Cinema (don't forget this came out in banner year of Bonnie and Clyde and The Graduate) both visually and thematically. They represent some of the finest and crispest black and white images ever put on screen, and emotionally they contribute greatly to the ennui and shallowness of post-modern America.
5. Speaking of great black and white images, you've got Conrad Hall largely to thank for those. Hall of course was one of the great cinematographers of all time, and In Cold Blood is one of the finest shot of all American movies.