There is a great deal of foreshadowing in the novel, particularly with regards to Dr. Sheppard’s guilt. For example, Dr. Sheppard’s simple statement that he was “considerably upset and worried” (p.1) suggests his profound involvement in the plot– he is not just upset by Mrs. Ferrars’ death, but by the implications that it will have on him.
Later on, he suggests that “as a professional man, I naturally aim at discretion” (p. 2), which on the surface seems an innocuous comment, but gains deeper significance given Dr. Sheppard’s actual involvement in the murder. Finally, Caroline’s accusation to her brother that he is a “precious old humbug” (p. 6) is a truer statement than even she knows. His utter duplicity throughout the novel will ultimately prove this insignificant comment to elucidate the novel’s greatest secret.
Additionally, Poirot’s story about the hypothetical man committing a crime foreshadows Dr. Sheppard's guilt, as does Caroline’s accusations that Dr. Sheppard is “as weak as water” (p. 199).