Imitation is the sincerest form of flattery...but when is it plagiarism?

I am a pretty good mimic.  I think it's because I pay attention and really focus on absorbing the style of whomever it is that I am listening to, or reading about.  In grade 8, one of our  assignments was to take a favourite poem and swap out the main topic for one of our own.  I have always loved doing this sort of thing because it feels like the hard work has already been done, and all I have to do is plug in an idea into a fully functioning mechanism.

All these years later I find I am exercising my writing muscle again and I'm inspired to do a repeat performance of the plug'n play poem.  This time though, I've got my sights set on an adapted screenplay of one of the book series I'm currently reading. It's all so clear in my head, right down to who the actors are in the lead roles (Shirley Maclaine and Julia Roberts)! 

Now, what I can't figure out is how I could ever convince the publisher who owns the rights to the series I'd like to adapt, to actually have me write the damned thing instead of the author.  Hmmm....research needed...how did all those Academy Award winners for Best Adapted Screenplay do it?  Methinks my ambitions are a wee bit too grandiose....