The Departed

The director and the editor often work together as far as switching around where certain scenes fall in the final edit. They'll switch things in this manner for concerns like tone, continuity, pacing, suspense, rhythm and wanting to reveal things later on or earlier on than originally written.

You've got to think of screenplays as blueprints. They're a starting point for a bigger idea.

Say you draw out the blueprints for your dream house - just as you want it. Then you start building the thing and you realise the kitchen needs a larger window or maybe another window altogether. You can't get granite so you switch to brick and due to local laws, you can't extend the garden as far as you thought so you opt for a larger front garden instead.

By the time the thing's built, it may not look like your original plans but the intention (to do the best you can with the material at hand) remains the same.

Screenplays work the same way. They're the I-Ching during production but by the time the film is released, although it may not entirely resemble the first draft, it (hopefully) remains a faithful interpretation of an original idea.

To George W.
Reading the Scripts of your favourite movies,whilst seeing how it all unfolds visually(on screen) ,is probably one of the best ways to learn about Screenwriting.
As Plot Device and Clock_ Work 9 have stated,the script(as does the movie) goes through numerous chops and changes.
Even if you have the Final draft(or revised Final Draft), you will still find that some Scripts/Screenplays have undergone major alterations.
The release Script is the only version, which is in it's completed state.
Though this is a production script,containing word perfect dialogue,music cues, ect.
The release script I imagine,would be the only Script/Screenplay which mirrors the final product(movie).
The Movie Collateral, Screenplay by Stuart Beattie,went through approximately 26 re-writes.
I have purchased particular DVD'S,which were refered to me(through my study), as having been fine examples of how a script should be written(structure ,format,ect).
Though many were close to the mark,most had glaring inconsistancies in dialogue and, when and where, the scenes took place.
Of course, this would have depended on which particular draft I was reading.
There was even a snippet(first ten pages) of one of the Screenplays in question, included in one book (from a famous Screenwriting Guru).
Yet the dialogue didn't match up,even though, the importance of this scene(particularly the dialogue), was being strongly emphasised.
In truth,this is nothing out of the ordinary.
Dialogue though essential(when written well),is not as important as getting the overal theme of the story expressed(visually) through the Action(descriptive) passages.
The famous screenwriter William Goldman mentioned in his book,(Adventures in The Screen Trade ),one of his Screenplays hit the screen(cinema) with only one line intact,from his origanal work,after other writers were called in to do re-writes.
He stated "It was the final line of the film".
Daniel.