As an actor, you're given a script and a character and told to go from there. You have to figure out why that character does what they do, and then you have to commit to that and sell it to an audience who will (hopefully) be engaged. And if they do not care about the character, they likely will not be engaged. This even goes for the school of thought that says audiences shouldn't be totally immersed and sympathetic -- but if they don't care about what they are seeing onstage, then why should they care enough to critique or think about what they are seeing?
So: you get a script and a character, someone who is not you, whose motives and actions may be totally incomprehensible. And you have to figure out why.
Writing and revising are a bit like that.
I'm currently rehauling That Novel I Wrote In High School* (yes, it is about as bad as it sounds) (and technically I wrote it in high school, finished it in college, and wrote the second book in college) and it's a lot like being given a script as an actor. I'm faced with the same sorts of questions, like...
Why do the characters do these things, apart from the fact that at the time I clearly wanted them to do it to advance plot?
Why does the plot move forward like it does?
Do characters' actions make sense?
A lot of the time, I found that there was no reason for the plot to move as it did (apart from that I wanted it to happen). Characters did not have compelling reasons or motives to act as they did. Their actions did not make sense beyond getting from A to B.
I thought for a long time that I was bad at plotting -- because all the above objections meant my plot made no sense.
But then I realized: it's not that my plot doesn't make sense. My plot doesn't make sense because my characters don't make sense.
I'm not bad at plot. I'm bad at character.
Luckily, I get to rewrite this "script." I can take the bones of the actions it contains and then write characters who would do those actions. And I can let those actions change as the characters develop.
Giving the characters things and people to care about in order to inform their motives has been difficult. Letting go of some of the basic assumptions I've held about this story and these characters for years has been difficult.
And -- if you'll permit me to ramble on about myself a bit more -- I'm glad I wrote that third book in college. I always heard that you should only work on one big project at a time. However, working on a different book helped me let go of this first project. It stopped being my "baby." Now that I'm not so attached, I can step back and rehaul it.
Rewriting an old project has taught me a lot about writing.