"Marlow is a person who follows life wherever it leads rather than one who tries to change it." On the contrary, Marlow only lets the trivial incidents slide along, seizing control of important matters. When Marlow decides that something is important, he does whatever it takes, "by hook or by crook," to get his way.

        Seeing a map of the Congo in a store window charmed Marlow into absolutely going to Africa. Once he had decided to go, nothing could deter him. That Marlow didn't hesitate to ask women, people he's already indicated contempt for, for help shows the strength of his determination.

        The voyage to Africa and most of the river trip are out of Marlow's control and incidental anyway. They are annoyances holding up Marlow's conversation with Kurtz, but only temporary, not worth the trouble of directly dealing with. Marlow doesn't concern himself because he is confident that the boat will get there itself within a reasonable amount of time. He waits.

        Kurtz's escape revulsed Marlow's Kurtz-integrity to a degree he couldn't afford. Marlow needed Kurtz to be consistent, to be a friend he could look to for armor against the global nightmare. If Kurtz had gone, Marlow would have gone mad. That being the case, Marlow spared no energy in stopping Kurtz, using the power of his arms and the power of his words: "You will be lost, utterly lost." Restraining Kurtz was the most important decision that Marlow made in the entire book and it was willful.

        Marlow logically rations out his energy to get what he truly needs and wants, letting everything else just flow along. He works hard to maintain his ship of reality-truth because he plans to sail it forever along the river of incidents. One small breach of his belief will sink the whole thing.