While the central story of Heart of Darkness is told by Marlow, a long-time sailor, it is in the form of an anecdote told to an unnamed Narrator, who frames the story with their voyage on the Nellie, a cruising yawl sailing down the Thames River in England. The narrator has little to do with the story itself, aside from being the person who relates it to the reader.
Between us there was, as I have already said somewhere, the bond of the sea. Besides holding our hearts together through long periods of separation, it had the effect of making us tolerant of each other's yarns—and even convictions.
(Conrad, Heart of Darkness, eNotes eText)
The Narrator does not interrupt the story, but listens and understands without judging Marlow's actions. His listening mimics that of the Reader, who sees the story through two layers of disconnect: Marlow to the narrator, and the narrator to the Reader. Despite this, the Narrator is changed by the story, looking out over the river and seeing not the comfortable water of his experience but a "heart of an immense darkness."
an unnamed person on docked boat, The Nellie