The first draft is the child's draft, where you let it all pour out and then let it romp all over the place, knowing that no one is going to see it and that you can shape it later. You just let this childlike part of you channel whatever voices and visions come through and onto the page. If one of the characters wants to say, "Well, so what, Mr. Poopy Pants?," you let her. No one is going to see it.
Now, practically even better news…All good writers write them. This is how they end up with good second drafts and terrific third drafts.