Writing for friends

So a few weeks ago, one of my friends told me about a project : do some kind of cheap indie zombie movie, just for the kicks.
Being a writer, i told him i was interested in writing it, for fun. And to add one line in my magical writer resume. You know, in case i ever get my ass to Hollywood and write a re-re-re-reboot of the Spiderman franchise for Sony.

But here is the problem with working for friends and/or family on this kind of project : you have to tell them who's boss the very first day. Hell, even before you take the gig.
Because those people won't see you as the writer (even if you are a real writer, with article, books and so on with your damn name on them). They see you as good ol'Bob, the guy they've known for 10 years.
So they will chime in. And you can't use every (sometimes even any) of their ideas. Because usually you can't fit them into whatever you have already written.

For example, a few months ago a high school friend came to me with a web series project about time travel. He pitched me an idea and wanted me (the writer) to take his basic idea (basically, 'something with a car-time machine in it') and turn it into a short abstract and a few lines about the people and the tech and so on.
So i spend a few hours doing my thing : writing basically what could be a first 5 to 10-minutes episode about some guy traveling in time. It's not Back to the future, but it's solid enough, with alternatives at every plot point for him to chose from.
And basically he took what i had done and decided that instead of the light-hearted homage to time-traveling movies i had written, the story should be some conceptual mambo-jambo about some guy that believed he was traveling through time, while he really doesn't. But for some reason he does not get that nothing happens.
And after that the project just died.

So on that new project, I'm keeping my independence.