The other side of editing
Over at Norman Hollyn-Wood, Norman wrote about how directors aren’t usually the right people to edit their films. Scenes aren’t usually the problem. It’s structure.
If you write, shoot and direct your film, you sometimes cannot keep the version of the film in your head that actually exists. You remember what you planned. You remember the versions you liked, the versions that the studio liked. You want to believe what you hoped for is there on screen.
The one time that I saw Robert Rodriguez’s “Once Upon a Time in Mexico,” I couldn’t understand what was going on. I remember repeated quick-fire exposition scenes. The plot seemed complex, and I’m usually the one that friends turn to to explain what was going on. Rodriguez may know how to edit a scene, but he was too close to the film to make the structure work. I think he thinks that there are scenes in the film that the rest of us never saw. He can understand the plot because he wrote the backstory and many unused scenes. I didn’t have access to any of that.
The editor is the one who’s job it is to keep track of all that. It is their skill to watch the film each time as if it is the first. The problem is education. If you think that teach people how to edit scenes is hard, just think about trying to teach people how to maintain the structure of whole films.
There are some director-editors who can watch their films as if it were the first time. I think Kevin Smith is a good editor for structure. That comes from his writing ability. He is a writer first, an editor second, and a director third. Not a bad order for the genres in which he works.
alex4D old blog 
I agree with this whole heartedly. A director/editor collaboration is one of the best relationships a film can have, especially if they are NOT the same person.
I choose to look at the 1978 Richard Donner SUPERMAN as a classic example of himself and editor Stuart Baird. These two fought over the edit like cats and dogs that are starving and angry. The result? An amazing film.
Look at Spielberg and Michael Kahn, 30+ years of working together (plus the little side gig with Verna Fields on JAWS). Anthony Minghella was almost defined by his relationship with Walter Murch as editor.
A writer/director is generally too close to their own work to have a perspective on what you brilliantly called “the film that exists”. I am dealing with that with a writer/director I am editing for now. We have radical ideas on how to deal with the film and the footage, but he is too married to what he “intended”, not what we have. I’m working with the producer to salvage the movie. It pains me greatly that the director wants to “Alan Smithee” the movie, but it will at least be finished.
It’s no surprise that you thought of Kevin Smith — editing is probably most closely related to writing of all of the disciplines. Still, in my experience, it’s hard for a director to be honest about his/her own film. It’s even worse when they’ve written the film as well.