The main narrative structure in Joseph Conrad's Heart of Darkness is that of the frame story, or the story told with an external "frame" that puts it in context and allows a separate view. This is commonly used in film to show flashbacks and other points of view; two films with this structure are Double Indemnity (1944) and Haywire (2011).
The frame story allows the unnamed "real" narrator to present Marlow's story from his own recollection. In the first part of the story, the narrator describes Marlow, who has unexpectedly spoken:
...Marlow was not typical (if his propensity to spin yarns be excepted), and to him the meaning of an episode was not inside like a kernel but outside, enveloping the tale which brought it out only as a glow brings out a haze, in the likeness of one of these misty halos that sometimes are made visible by the spectral illumination of moonshine.
His remark did not seem at all surprising. It was just like Marlow. It was accepted in silence. No one took the trouble to grunt even; and presently he said, very slow--
"I was thinking of very old times, when the Romans first came here..."
(Conrad, Heart of Darkness, eNotes eText)
This allows the narrator two levels of disconnect between the tale of Kurtz and its telling; he does not have Marlow's intimate view, but can see the outward discomfort that the telling has. This also allows the "unreliable narrator" to come through, as the narrator makes no effort to comment or fact-check Marlow's story, and so the reader has no idea how much Marlow is adding, embellishing, or diminishing.