In the penultimate chapter of The Murder of Roger Ackroyd, Hercule Poirot compliments Dr. Sheppard on his cleverness. Even in the "Apologia" which serves as Sheppard's confession and suicide note, the narrator cannot resist congratulating himself in a similar matter:
I am rather pleased with myself as a writer.
He goes on to show how neatly he managed to tell the truth and nothing but the truth, while avoiding the whole truth, for the entire narrative.
Sheppard's self-congratulation and Poirot's appreciation for how cleverly the murder was committed are both ways in which Christie draws the reader's attention to her own ingenuity. Short of having the detective himself commit murder (something she was eventually to do in another book), the most unlikely conclusion the reader can imagine is for the narrator who has been guiding them through the story from the first page to emerge as the culprit.
In order to make the structure of such a story work, the author must be scrupulously fair in giving the murderer no untoward advantage. Sheppard's confession also functions as a way of recapping the clues Christie dropped along the way and a summary of the difficulties Sheppard faced, both in committing the murder and in concealing it from the reader. Sheppard points out that one of these difficulties was the constant surveillance of his sister, Caroline, whose curious nature and tendency to gossip have caused him the greatest unease.