Over the weekend, I participated in a horse show (don't worry, this will be relevant in a minute). Like nearly every one who rides or participates in any sport of any kind, I left the ring feeling like, "that was ok, but I could have done so much better." It's a pretty common feeling -- I should have thrown the ball farther, or I could have played that poker hand just a little bit smarter, etc. That's all fine and good in certain aspects of your life, but writing isn't one of them.
Don't get me wrong; there is a time for rough drafts and "Hail Mary" takes on an idea or a script. The adage about writing being mostly re-writing is as about as true as any saying can be. Heck, I've already re-written this blog post 5 times and I'm not even close to being done. Try out a version, give it to your writing group to get some feedback, play with the characters to see what feels grounded and real.
However, the script/pages/pitch that you hand in to a producer, agent or whoever that final destination for it might be, better be the best darn version you have in you. Period. End of story. If you need to take a couple more days to feel like this is the best version you can give, do it. Because the odds that you'll get a second read out of someone are slim to none.
The only exception to that rule is if the person/studio/network you're giving the project to has a vested interest in reading it and giving you notes until they think it's right. Guess what? If they feel like the draft you gave them was anything less than the best you can give, they're going to be peeved. Even if you're doing work on an if/come basis for someone, what they are investing is their valuable time and they will be equally peeved if it feels like you're wasting it with half-baked plots or undeveloped characters.
What CAN happen is that you turn your draft in, a draft you've sweated blood over, and the recipient responds with notes. Maybe a ton of notes. They liked your general concept, but the specific plot doesn't work. As the writer, you can discuss it with them, listen to their concerns and engage in a dialog about how to fix it. What you CAN'T do is say, "I hear you, let me try to make that better".
I know, it sounds like a good thing to say, right? You think you're telling them, "I'll do a better version of that story or character now that I know what you want." But you're really saying (whether you know it or not) is, "I didn't give you my best work because I wasn't sure you'd like that idea. Now that I hear that you don't like it, I'll do a better job of writing that exact same thing, certain that when I give it 100% you'll see that I was right and love this as much as I do."
BZZZZ! Sorry, wrong answer. You had your shot before you turned in that first draft. If there were things you weren't certain about, call the people you're working with. Really listen to them, don't do that half-listen thing you do when you ask someone a question to be polite and really don't care what the answer is. Once you've turned it in and the person tells you an idea, plot or character isn't working, don't keep hammering at it with the idea that if you write it better they'll get it. Right or wrong, for better or worse, not only will they not get it, they may very well get pissed that the very thing they didn't like it back on the page in front of them again.
Be bold, be daring, go for the high fences as you write and rewrite, but be sure that whatever you give to the network or studio is the absolutely best you can deliver. If they don't like it, file it away in your "ideas to come back to" file and come up with something new that will work for them. Because promising to write the same idea but better next time is like promising sex will be better next time; it leaves your partner wondering what you were holding out for in the first place.