Dramatic irony occurs when the readers of a book know what the characters do not. In Capote's narrative, we as readers learn early on that the Clutter family—Herb, Bonnie, and their teenage children Nancy, 16, and Kenyon, 15—will be murdered with "four shot gun blasts that, all told, ended six [the six includes the two murderers who are executed for the crime] lives."
Dramatic irony abounds as the Clutters go about their ordinary lives with no idea at all that Dick and Perry, recently released from prison, are heading across Kansas and will soon murder them, mostly at Dick's behest. Our tension as readers grows higher as the Clutters go about their daily business; for example, in Nancy's case, showing a friend how to bake a cherry pie or worrying about her dating relationship with Bobby while unbeknownst to them—but known to us—Dick and Perry are coming ever closer.
Chapter 1 talks about the murders before the family is actually killed. For example, we as readers get to hear Dick say he wants to get "plenty of hair on them-those walls." Obviously, the Clutters, unlike us, have no idea this man is coming.
Throughout part 1, Capote will mention that the Clutters are doing an activity for the last time and that this is the last day of their lives. He keeps in front of us the fact that the murder of the Clutters will soon occur without them having one hint.