The Secret Sharer

Being a story driven by the voice of the protagonist, Conrad has the task of shaping the sentences in his prose in such a way that lets us know who this captain is.
His use of high style is revealing of the man’s character. He’s educated (he uses big words) but he’s also a romantic, one that dwells on abstractions as much as he does in action (in theme as much as in plot).
The story opens with a description of the setting that echoes throughout the story. The captain describes the sea as solid. Latter, he says:

“And suddenly I rejoiced in the great security of the sea as compared with the unrest of the land, in my choice of that untempted life presenting no disquieting problems, invested with an elementary moral beauty by the absolute straightforwardness of its appeal and by the singleness of its purpose.”

The character is unsure of what to do or what to say in matters of human interaction, but finds comfort in the “straightforwardness” of the sea.
After being burdened with matters of the crew, he suddenly has this change in rhythm and tone, where the pace of the prose picks up with great emotion. First, he makes a distinction between what he finds safe and what he does not, the next clause is self-reassuring (I know I like the sea, I picked it as my career choice), then he glorifies it in a way that is not exclusively poetic, but rather helpful in deciphering who this man is. We are discovering it with him, for this sentence also serves as a moment of realization for the character, (an A-ha! Moment, if you will.)
But in matters of “the land”, this imperfection of narration- this learning as he speaks device is not romantic or poetic, it’s neurotic.

“My action might have made me appear eccentric. Goodness only knew how that absurdly whiskered mate would "account" for my conduct, and what the whole ship thought of that informality of their new captain. I was vexed with myself.”

These collection of short sentences that aren’t nearly as poetic as the majestic, certain one about the sureness of the sea. These are shaky in emotion and even a bit snappy (“that absurdly whiskered mate”, having “account” surrounded by quotations). The segments ends with a short, troubled little sentence- a far cry from the rhythmic, tranquil longer one he has when calm.

Unfiltered self-reflection carries the voice.

“I should have gathered from this that he was young; indeed, it is only the young who are ever confronted by such clear issues. But at the time it was pure intuition on my part. A mysterious communication was established already between us two — in the face of that silent, darkened tropical sea. I was young, too; young enough to make no comment.”

Conrad has the captain dubbing his new friend as “my double” in his mind. This man has come to act as a mirror for the narrator, something that he’s discovering as he tells us. The dash interrupts his thought, as if he’s taking time to ponder. This segments works that way. First he’s being clever and calling this man young for traits he observes in him, then he says how alike they were, and then the realization that he too was young.