Filmmaking on paper – that's screenwriting. And as the writer, you're the first director, the first cinematographer, the first set designer, the first... everything. You see it all. But because you have the whole thing in your head, there's a tendency to try and stuff it all in – every single detail. You can get away with that in a novel, but not with screenwriting. Script economy is essential to the screenplay: you must kill your babies, maximize white space, start scenes at the last possible moment, and get out early. However, all this can be difficult if you have too much story to tell.
But how do you know if you have too much story for your movie? Think of it this way. Imagine going to the Louvre in Paris and trying to see everything in one day. You most likely would walk out feeling like you saw nothing at all; however, if you went in and spent two hours just examining Renaissance art, you would walk away with a wonderful, enriching, and maybe even life changing experience. Movies work the same way.
So, for example, if your screenplay is on a famous historical figure - unless it's a mini-series and you have 8.5 hours to fill as was the case with HBO's seven episode 2008 mini-series John Adams - it would be a HUGE MISTAKE to try and jam in the entire life story in two hours. Therefore, when writing the two-hour version of John Adams, you should pick one major event in the man's life and focus on that - really get into it and make us experience it with all our senses.
Too much story is a problem. So always try to look at one event within the larger story context, and tell only that event. You don't have to lose the primary characters, and you're able to tell a much more detailed story in a more specific period in their lives. The backstory of the characters can still come out during the event - through conflict, arguments, even through humor.
Just remember, don't have too much story for your screenplay. The best - and often academy award winning - stories are usually the simplest seeming ones.