I would like to depart from the tendency in looking through Marlow to Conrad rather than at Marlow as an object of interest in his own right. Reading Marlow’s behavior as inwardly motivated (as a character in the book rather than a mouthpiece offering little more than narrative distance for Conrad), would lend a pyschological dimension to his narrative machinations.
A note-worthy observation regarding Marlow would be his obsessive need for narration and categorization. He has an obsession for “the idea of Kurtz” in Heart of Darkness, just as Jim’s story haunts him in Lord Jim. This obsession acts as a lubricant in the narrative motion of the texts in question and also highlights the subjective nature of narration and truth-telling (little ‘t’) in modern texts.
In Lord Jim, Marlow becomes a “receptacle of confessions,” fascinated (to the point of obsession) with Jim’s story as seen by his collecting of various narrative sources. Marlow becomes a kind of Ancient Mariner figure, whose role is to compulsively record and repeat what, by extension, haunts him. He sees this obsession as a curse (“diabolical”) for which he admits “is a weakness of (his)”. This motif runs through the text with Marlow’s compulsion to narrate (or rather, form narrative threads) is likened to debauchery and addiction (drink, women) repeatedly.
If we are to see Marlow as a cursed Ancient Mariner-type figure, then we could see the act of narration (the framing of the little ‘t’) being one born out of an unknown compulsion. Just as Coleridge’s Ancient Mariner told his tale as a for redemption (out of guilt), Marlow’s “diabolical” compulsion may be one spawned from the basic human desire for the capital ‘T’ Truth, while the narrative itself is framed through the subjective (hence unreliable) view-point of Marlow, and as such can only take the form of the little ‘t’. Perhaps what is diabolical is not so much the compulsion to narrate, but the inadequacy of the human frame of reference to fulfil the desire for the Truth in its limited form of representation.