Heart of Darkness is styled as a story told by Marlow to a group of his fellow sailors. This allows Marlow a certain amount of embellishment for dramatic purposes, and also removes the burden of proof from the "real" narrator, who only comments at the beginning and end. Atmosphere is created by Marlow's detailed imagery and his discussion of how the Congo affected his mind; he uses a lot of metaphors and symbolism in telling his story.
The dusk came gliding into it long before the sun had set. The current ran smooth and swift, but a dumb immobility sat on the banks. The living trees, lashed together by the creepers and every living bush of the undergrowth, might have been changed into stone...
(Conrad, Heart of Darkness, gutenberg.org)
This passage is typical of Marlow's exposition; he speaks more of the feelings and emotions created by circumstances than of the actual events. Even when recounting the native attack, he spends more time on his own feelings than on the actual event; the effect it has on his mind is, he feels, more important in the telling than who was killed or how long it took. This allows the reader to feel more of Marlow's emotional reactions throughout, instead of simply reading the account of his journey.