One of the most common causes of weak or sludgy plots is the confusion of internal conflict with external conflict.
Writers who focus on character love sittin’ and thinkin’ scenes where protagonists look out windows and contemplate their pasts or ponder the meaning of reality, the battle raging within them over conflicting needs and values. That’s internal conflict.
Writers who focus on plot love action scenes, scenes where two people duke it out over conflicting needs and values, the battle raging in the external world as physical action. That’s external conflict.
But just as a story needs both character and plot, it needs both internal and external conflict. A story that has only one is weakened by its lack of conflict depth.
External: Raiders of the Lost Ark has a very simple external conflict: both Indiana Jones and the Nazi Belloq are after the Ark of the Covenant, a religious artifact with great power that could decide who wins WWII.
Internal: Raiders has a much more complex internal conflict: Indiana Jones is passionate about religious artifacts that he steals but is himself empty spiritually. He’s essentially trying to pay off his spiritual bankruptcy with stolen goods.
External and Internal: The external conflict forces Jones to accept that there are things he cannot understand and must just believe; because he resolves his internal struggle in favor of belief, he can say at the end, “Don’t look,” because he knows there are things that are not meant for men to see. Because he knows that, he survives and takes the Ark.
Without the external conflict, Raiders of the Ark would be three hours of Harrison Ford thinking a lot. Without the internal conflict, Raiders would be just another action movie. With the two combined so that the events of the external plot provide the resolution for the internal conflict, Raiders is one of the best movies of all time.
You need both external and internal conflict to fuel a story with depth that resonates with the reader.