It may sound like an admirable quality--striving for perfection--but all too often it means the person never finishes writing his book or her screenplay because it will never be perfect.
Some perfectionists give up in the middle because they can tell their project will not live up to their high expectations.
A powerful solution came up in an article by writer Denise Gosnell, who said she learned it from a coach named Dan Sulliven.
In a nutshell, the idea is to get quickly to the point where you are 80% satisfied. Don't keep going back and trying to get everything right; instead, forge ahead and acknowledge that there will still be work to be done when the first draft is finished.
If you're a very strict perfectionist you may have to set the bar lower--say to 70%--in order to be able to let go of the impulse to revise repeatedly as you go along.
Once you've done that, evaluate what improvements are needed. Make those to the same level of satisfaction. For instance, let's say you realize you need to make your protagonist more likeable. You make some changes in that direction. When you think you are most of the way (80%) there, stop and go on to the next thing that needs to be changed.
Then repeat the process once more. By that point it's likely that you're as close as you're going to get to making the project as good as you can get it on your own. That will be the time to have it read by a trusted friend or colleague who you know gives constructive and honest feedback (more than one, if possible).
If they come up with issues you feel are on target, repeat the process once more, maybe twice. Then send the project out into the marketplace...even though it still won't be perfect.
(Friendly guidance from idea through to the still slightly imperfect finish is what you'll find in my book, Your Writing Coach, published by Nicholas Brealey. You can get it right now from Amazon or your other favorite book seller.)