I've written some X-Men fanfic, usually places where I felt the "soap opera" nature missed a couple of scenes. 

And, as a writer, it's a shortcut.  I can write a character's name, and many of the character's traits are already  established, just there.  I don't need to spend any time SHOWING that Cyclops is a stiff control-freak; I don't need to TELL the reader that.  I can just make him walk in to the scene when I need an authority figure to frown a lot.  There can be more to it than that, but IMNSHO, a lot of fanfic writers use that crutch to help work out their own plotting, pacing, and other craft issues without having to work on characterization at the same time.

And there's a built in audience.  It's terrifying to write something, and ask someone else to read it and hope that they like it.  If you write a fanfic, you're guaranteed readers.

-FredG

I'll recommend Screenplay, by Syd Field, as one of my favorite resources.  It doesn't help me with plots, characters, or word choice, but it helps with timing and pacing. 

I know that I should have about twice as much middle as end and beginning.
To misquote Checkov, if I want to have a gun go off in act three, I need to write it into the room in act one. I know that I need to sometimes think of the chapters as a series of sequences strung together.

By knowing what's important to the Imginary Film Version of my story, I know what to put (and not put) in my written version.

-FredG