It’s very easy to get carried away while writing a story especially if you haven’t written an outline.  The simplest thing that can be overlooked is logic.  Does what you’ve written make logical sense?

There are a few key elements to consider in terms of logic:

• Timeline: Does everything you have happening in your story make sense for the time in which it takes place?  It’s important to consider realistically how long it will take someone to get across town or to accomplish a task.  If your story has a ticking clock, make sure you don’t get your hero accomplishing ridiculous feats in super-fast time.
• Show Rules: Just because it’s a movie, it doesn’t give you license to break the boundaries of realism.  This is what’s called suspension of disbelief.  If in the world you created, people can fly, that’s fine provided you establish the ‘rules’ for the world they inhabit early on and follow them.
• Consistent Behaviour: Do your characters behave consistently?  They can certainly do things that are out of character, but most people behave predictably especially in stories.  If, for example, your hero is polite and well-mannered then she’s totally inappropriate, this behaviour must be justified or it’ll seem inconsistent. 
• Simple Human Habits: There are certain things people would do without thinking that are often neglected in scripts.  If someone drops a glass, people instinctively clean it up.  If someone sneezes, someone says ‘bless you’.  If your characters don’t do this there must be a reason.
• Overlooking the Obvious: This often happens when you as the writer feel you have to get your character to do something for visual effect and you overlook the obvious to get him there.  For example, your protagonist is a murderer who has just killed someone driving a car, but you have him walking down the road; why is he walking when he could have stolen the car? 
• Coincidences: Do things just happen too easily?  Does your entire plot hinge on a coincidence?  Fix it now!  If it seems far-fetched, no one will believe it.  Remember Runaway Jury by John Grisham?  The entire story hinged on the fact that two jurors happened to be put on the very case they wanted.  This never happens in real life.  Jury duty is totally random.  You can’t pick cases you want to be on.   

Everything that happens in your story should be a direct result of the conflict arising between two people.  When you remain true to the characters and the story you’re telling, it will flow naturally and these inconsistencies will fall away. 

At times, however, we get lost in the story and fail to see our own mistakes.  Make sure you show your script to someone who will note these flaws in logic and not excuse them as casualties in your script because ‘it’s just a movie’.  If your movie isn’t based in reality, who’s going to care?  There’s nothing at stake if there’s no reality because some giant space ship can come down from the sky at the last minute and save them all.