One topic that can be developed well is that which deals with the strongest element of Joseph Conrad's story.  About his major theme, Conrad himself stated,

...the solidarity in mysterious origin, in toil, in joy, in hope, in uncertain fate, which binds men to each other and all mankind to the visible world. 

When the untried young captain takes aboard his ship the escapee from the Sephora, Leggatt, he unites with his doppelganger, or double, who is his darker side.  Yet, he benefits from his "secret sharer" because he learns to be more aggressive and assertive.  Even his act of helping Leggatt flee from Captain Archbold by taking the ship close enough to the island of Koh-ring establishes the captain's skill in navigation as he directs the ship dangerously close, but safely, near shore, an act that earns the respect of the crew.  Thus, this "mysterious solidarity" of men becomes very meaningful for the young captain.  That is, in Contrad's narrator's words, "meaning depends upon sharing."