Utterson, whose perspective the narrator follows the most closely, is frequently the last person to know of crucial developments in the story. Often, these events are recounted in flashback, adding to the narrative’s suspense. One of the most important flashbacks comes at the end, when Lanyon’s letter reveals the truth about Hyde. It begins:
On the ninth of January, now four days ago, I received by the evening delivery a registered envelope, addressed in the hand of my colleague and old school-companion, Henry Jekyll.
The letter begins by situating both the events it recounts, and even its own writing, in time (“on the ninth of January, now four days ago”). Its tone feels official, as if it were a legal deposition; given that Utterson, Lanyon, and Jekyll know one another intimately, Lanyon could simply say he received a letter “from Henry,” but he opts instead for an extremely formal introduction that identifies his relationship to Henry in specific terms (“in the hand of my colleague and old school-companion”).
This could be in part because Lanyon’s letter is going to assign responsibility for criminal activity to Henry (as Hyde). Given Lanyon's request that the letter should not be read until both he and Jekyll are dead, the reader knows Lanyon never hoped or intended that the letter would make it into the hands of law enforcement. But the lucid, exacting style of the letter, which strives to leave nothing out or unaccounted for, helps this flashback achieve its key function of resolving the many mysteries within the story effectively, to the end of satisfying readers who have been long waiting for resolution.
The appearance of this letter literally halts the events of Utterson’s narrative. Utterson tells Poole not to call the police until he returns having read them, and the body of Hyde still lies on the floor of Jekyll’s home when Utterson leaves. It is expected that the letters will finally reveal the association between Hyde and Jekyll, and the secret that killed Lanyon.
Placing this information at the very end of the novel—in the form of flashbacks that bring the reader back to the moment of revelation—heightens the drama and anticipation of these revelations. The anticipation of Hyde’s eventual unmasking actually helps propel the story forward, as Utterson hunts for and fails to guess the extraordinary truth. What’s more, flashbacks like this one allow Stevenson more control over the narrative than if it merely ran chronologically. If it is more interesting, helpful, or constructive for a key event to occur in flashback, several days after where it falls chronologically, Stevenson can impose that restriction on both Utterson and the reader.