Christie's reputation as "The Queen of Crime" was built upon the large number of classic motifs that she introduced, or for which she provided the most famous example. Christie built these tropes into what is now considered classic mystery structure: a murder is committed, there are multiple suspects who are all concealing secrets, and the detective gradually uncovers these secrets over the course of the story, discovering the most shocking twists towards the end. Culprits in Christie's mysteries have included children, policemen, narrators, already deceased individuals, and sometimes comprise no known suspects (And Then There Were None) or all of the suspects (Murder on the Orient Express).
At the end, in a Christie hallmark, the detective usually gathers the surviving suspects into one room, explains the course of his deductive reasoning, and reveals the guilty party, although there are exceptions in which it is left to the guilty party to explain all (such as And Then There Were None and Endless Night, both rather nihilistic in nature).
Christie allows some culprits to escape earthly justice for a variety of reasons, such as the passage of time (retrospective cases), in which the most important characters have already died, or by active prescription. Such cases include The Witness for the Prosecution, Murder on the Orient Express, The Man in the Brown Suit, Elephants Can Remember, and The Unexpected Guest. There are instances in which a killer is not brought to justice in the legal sense but does die as a direct result of his plot, sometimes by his own hand at the direction or with the collusion of the detective (usually Hercule Poirot). This occurs in The Murder of Roger Ackroyd, Death on the Nile, Dumb Witness, Crooked House, The Hollow, The Mirror Crack'd from Side to Side, Cat Among the Pigeons, Peril at End House, Nemesis, Appointment with Death, The Secret Adversary, and Curtain. In the last of these (Curtain), no fewer than three culprits die during the course of the story.
In The A.B.C. Murders, the murderer has killed four innocent people and attempted to frame an unstable man for the crimes. Hercule Poirot, however, prevents this easy way out, ensuring a trial and hanging. In And Then There Were None, the killer's own death is intrinsic to the plot; the red herring is when and how the killer actually died. However, stage, film, and television productions of some of these mysteries were traditionally sanitized with the culprits not evading some form of justice, for a variety of reasons  e.g., censors, plot clarity, and Christie's own changing tastes. (When Christie adapted Witness for the Prosecution into a stage play, she lengthened the ending so that the murderer was also killed; this format was followed in film and television productions, most famously the Charles Laughton/Marlene Dietrich film.) In Death Comes as the End, set in ancient Egypt, the culprit is killed in the act before he can claim another victim by one of the few surviving characters.
In some stories, the question remains unresolved of whether formal justice will ever be delivered, such as Five Little Pigs and Endless Night. According to P. D. James, Christie often, but not always, made the unlikeliest character the guilty party. Savvy readers could sometimes identify the culprit by simply identifying the least likely suspect.
On an edition of Desert Island Discs in 2007, Brian Aldiss claimed that Christie had told him that she wrote her books up to the last chapter, then decided who the most unlikely suspect was, after which she would go back and make the necessary changes to "frame" that person. However, John Curran's Agatha Christie: The Secret Notebooks describes different working methods for every book in Christie's bibliography, contradicting the claim by Aldiss.
Output: What is the first step in the classic mystery structure Christie is known for?