I think the most important thing is to, without belligerence, stand up for what want. Argue compellingly if someone tries to change your script. Yeah, legally they can if they want to. But rather than give up, as some of the writers do, and just wail about how your script got rewritten, it's much more difficult - but well within the realm of possibility - to argue very sincerely, calmly, and reasonably from your point of view, such that the director or the producer might decide, "All right, let's do it that way."
