There is a tremendous difference between having a character walk through a set of plot elements, and letting that same character crash cleanly through them because he/she is properly motivated.
I was reading a younger writer's efforts last year, and this extreme example has stayed with me: The main hero was standing in the middle of the room, discussing something urgent with three other people. He turned to rip a closet door open, dragging somebody out of the closet with a casual one-handed grab. He then presented his trophy in a grand (TaDaaaah!) sort of a moment.
Well, okay. Why did he rip the door open though? There was no foreshadowing. Did he hear something? Was there some clue I missed three chapters back? Is it explained in a sequel? Does he just hate closet doors? Is it one of those nauseating Star Trek time warp thingies? Whut?
Pulling people out of a closet for no apparent reason is on the micro side of the story. The macro parallel is more often just as incoherent, obtuse, or just plain ill advised plot line. If a character is not clearly motivated to do something, then the reader may slowly lose interest. It may make perfect sense to you as the writer, but you must convey what is going on in the inner spaces of the character's mind or show us how he made those choices.
If a character is written so that we understand his/her motivation, the plot is much easier to navigate. When the character meets those decision points the choices are clearer. My view is that the plot is passive – the result of whatever motivated the character to make a choice. The plot is not instructions like in a Broadway play that requires the character to be in Vienna by Thursday. The motivation for getting to Vienna by Thursday may be that if he doesn't go, then his girlfriend is killed. Or he loses the business deal. Or he has to face Darth Vader's evil twin Zippy with just a plastic light saber and harsh language.
So how do we motivate a character? Turn the question around – what motivates YOU?
Greed, lust, love and vengeance seem to have pretty high marks for repeatability. I'll be circling back to this topic when I'm in 'motivation mode' and you will see my views on how to get your characters really moving – organically. If it makes no sense for your character to do what he/she is doing from an objective standpoint, then you are forcing plot, not letting the character decide what needs to be done.
No, these are not the final words on the subject, but hopefully enough words to get some dialog going!