An important chapter in The Murder of Roger Ackroyd is chapter 5, called "Dinner at Fernly." It is during this chapter that Roger Ackroyd is killed.
This mystery is famous because the first person narrative is told by the killer, who fails to reveal that he is the murderer. From a literary point of view, Christie deceives her readers through the use of the literary convention of elision; this is a literary device in which writers leave things out. Christie plays on the fact that writers almost universally elide parts of their stories. This is because if every detail, such as everyone's bathroom break, were included, we would be so bogged down we would never get through a novel.
Therefore, we trust our narrators to provide us with the important facts in a narrative. Dr. Sheppard, the murder, is a uniquely unreliable narrator. While we are trusting him to make sure we have the important facts, he is leaving them out. For example, he states the following as he looks in a curio cabinet before dinner:
There were one or two pieces of old silver, a baby shoe belonging to King Charles the First, some Chinese jade figures, and quite a number of African implements and curios.
He fails to mention that one of the pieces of old silver was a knife. He then states:
I was still bending over the open silver table when Flora Ackroyd came into the room.
He elides the fact that he has slipped the knife out of the silver table to use as a murder weapon.
We are given enough information to know that Sheppard is in the room by himself for long enough to have taken something from the curio cabinet. We learn not too long after this that an antique knife was the murder weapon: Christie leaves it to the reader to put two and two together.
We don't, because we generally trust a convention in which the narrator of a murder mystery is not the murderer. Sheppard presents himself as trustworthy, such as by offering quite a number of specific details about the evening in question. At the same time he leaves out the most important facts.
Christie's straightforward writing style also deceives us. Because of it we have no cognizance of how many tricks she is hiding up her sleeve.