If there's one mistake first-time writers make, it's over-writing. I'm not just talking about writing dialogue and speeches that go on too long. Often they'll write too many scenes to drive home a particular point. They won't just start with Joe on a train just as it pulls into the station in his hometown, they show Joe packing in his hotel room. Then they show him checking out of his hotel and asking for directions to the train station. Then we see him buy his ticket for home, then an entire scene of him getting on the train and watching the world go by, and THEN at last, we see him arrive at home and the story finally starts.
So what could have taken one scene and one page ends up consuming ten pages and ten minutes of screentime.
Guys like me often harp on the "Start a scene as late as possible, leave a scene as soon as possible" rule, but I'd also encourage writers to think hard about if a particular scene is even necessary. We don't always need to see A to B to C to D. Sometimes you can show us A, cut to D and leave B and C as points that we can fill in for ourselves.
If you've got a script that's 128 pages and you're convinced you can't cut any more, you're wrong. I just read a script this week that was 105 pages and jam-packed with story and character development. It was a fast moving plot that seemed to contain twice the twists and turns that I see in the first-timer scripts I sometimes end up reading.
It's amazing the blanks that an audience is capable of filling in. Trust them. Leave something to their imagination. See how much you can cut before they get confused. And true, you won't always know. If you watch DVD deleted scenes, you'll often see scenes that were cut for redundancy. Even the pros have trouble with this, but that doesn't mean it's not worth the effort to master.