You have to make it track emotionally. But in the end, rewriting is harder than writing. It's hard when the rewrite is scrapped or is cherry-picked for choice moments. It's like being invited to someone's house for a week and rifling through their drawers. But sometimes, after two or three years and three or four rewrites, the original writer can get ground down and fed up.
Personalities can get flinty.
The memos take on an edgy tone. In comes the script doctor. They say it's two weeks' work on one character. Four months later, you're still on the picture. The funny thing is that, with all these smart people exerting all their energy fine-tuning characters and dialogue, most movies are still junk.