Along those lines, if you turned to others to help you produce your script, those backing you financially will have a say in what is done.
Also, any actors that are going to be in a film also have a say. An actor can read a script and feel that it is good, but it needs some changes. As the screenwriter, it will be your job to accomodate them, as people go to movies in part to see their favorite actors.
Generally, in the real paying film industry, everything is compartmentalized. Screenwriting is a totally different animal from directing.
In the indie world, people will wear many different hats. But there, I have known screenwriters/directors/producers/lead actors who filmed their own movies out of pocket and not only got no distribution, but no film festival would show them.
And as Writing Again just said, publishing companies will have changes made if they think it will help sales.
Writing won't be all about you, if you want to produce a product that other people will see or read.