I think the "Drivers and Passengers" part of the Theory Book sums it up pretty well:
"We can easily see how these Archetypal pairs represent a broad analogy to a human mind dealing with a problem. The Protagonist represents the desire to work at resolving the problem. Its Dynamic Pair, the Antagonist represents the desire to let the problem grow. As with the Archetypal Characters, we all face an internal battle between making decisions based upon Reason or upon Emotion. [...]"
Characters help to form the argument of the story with their characteristics. The Protagonist might influence others to consider solving the problem, while the Skeptic might say it's a bad idea, and want the Protagonist to give up on the Story Goal.
Archetypes also move the story forward or hinder its progression. The Drivers will feel to be the prime movers, while the Passengers might have a big influence on the Drivers’ choices and approaches.
I guess the Decision characteristics are more about the influence the character has, while the Action characteristics are related to what the character mostly does.