To get the authenticity he wanted, Richard Brooks filmed in all the actual locations including the Clutter house (where the murders took place) and the actual courtroom (6 of the actual jurors were used). Even Nancy Clutter's horse Babe was used in a few scenes. The actual gallows at the Kansas State Penitentiary were used for filming the executions, however, in a 2002 interview, Charles McAtee (who was State Corrections Director for Kansas in the 1960's), clarified the hangman in the film was an actor, not the real deal.
In two scenes Robert Blake's character makes a reference to The Treasure of the Sierra Madre. Blake played the paperboy who sold the winning lottery ticket to Humphrey Bogart in that classic movie. However, despite the fact that many people believe it was written into the script because of Blake it wasn't. According to Truman Capote it was Perry Smith's favorite movie.
The "Jenson"/"Narrator" characters are based on the author himself, Truman Capote. Capote went to Kansas soon after the murders to cover the manhunt and to interview those who knew the Clutter family. After the apprehension and conviction of killers Perry Smith and Dick Hickock, Capote became a major part of the killers' lives while they were on death row, forming a particularly close bond with Smith. Smith gave most of his belongings - drawings, books - to Capote. Capote was present at the executions and witnessed the carrying-out of Hickock's sentence, but couldn't bear to watch Smith die, and left the room before he was brought in.
During the scene where Perry Smith (Robert Blake) is summing up his life, the rain on the window is reflected on Blake's face so that it looks like he's shedding tears, and the effect was lauded by many reviewers. That effect wasn't planned - the set was hot that day and a fan was being used near the simulated rain; it accidentally blew the water against the window, resulting in the shadows of the falling "rain" passing over Blake's face.
The producers of the film originally wanted Judge Roland Tate, the actual judge from the trial, to play himself in the film. Judge Tate died shortly before photography and a call was issued for a suitable replacement. Local auctioneer and realtor John Collins was cast, and appears in the film.