The lines between copycat and new

Nearly every time I read a book or watch a movie that really grips my imagination, I often relive it in my head numerous times.  This can go on for hours or even weeks after the movie or book makes its first impression.  Sometimes I'll include myself as one of the existing characters, but often I'll create an entirely new character to be me, and incorporate my "character" into the plot.

Does anyone else do this?  I've done it my whole life, and this is one of the things that attracted me to Mercedes Lackey when I was a teenager.  I was reading one of her novels, Arrows of the Queen, which is part of the larger Heralds of Valdemar series.  I'd already read a few novels from the series, out of order, before reading this one.  As the book opens, the main character is reading a story and imagining that she is part of the story.  I recognized this right away because the history tale the character was reading came directly from one of the other novels I'd read.  My immediate reaction was "I do that!!!" and I was hooked. 

Most of these stories I play out in my head can never be written down (or rather, they aren't worth it) because they take place in somebody else's universe.  It doesn't matter how enthused I am about the idea, I can't sell it if it's somebody else's story rebranded with my character.  I don't even think about doing such a thing, because I don't want to spend my precious writing time penning something that's destined for the fan fiction boards, if even that.  Every now and then, though, a story mutates so much that it becomes something of its own.
This happened to me recently.  It's a story based on an old science fiction film, but its catalyst was really just one small moment of the film.  One scene stuck in my head, mutated and evolved, and has gone off in some very wild directions.  This story has actually been with me for several years, off and on, as something my imagination plays with when I get bored--especially while driving or doing dishes, or other times when I'm typically alone with my thoughts.  Each time I revisit it, it evolves some more.  It gripped my mind again recently, and I found it was interfering with editing my novel because I was having a hard time putting it aside.

Then it hit me:  this has changed so much, it really is a story of its own now.  So why not write it down?  This'll be a unique chance to harness the creative energy that normally just keeps me amused throughout the day.  I jumped into writing, or at least summarizing, and I haven't looked back.  And it's going quite fast.  I think it's probably going to be novel length, and it has the advantage that I'm very excited about it right now.

I don't want to write the new one completely right now, but I'm trying to get the plot and all the scenes I've already worked out in my head summarized before I lose any details or my enthusiasm.  It's very disorganized, but still rewarding.  The only difficulty is that this has made me put my first novel on hold.  I suppose it amounts to stalling.  I'm still a bit nervous about all the editing something as long as a novel entails, so it's easy for me to justify finding other things to do.

Well, it's good to know what the next project will be, at any rate.  And I really am curious--does anyone else write themselves into good books and movies?