Voice Over. Does it really improve your screenplay? Is it really the best way to bring out backstory? Does it really maximize how to reveal crucial information? No. Not really. Not really at all.
As a rule, Voice Over should be a last resort. Because it’s almost always a mistake, and for beginning writers it is always a mistake.
And I know what you're thinking, "But there are so many great movies that use it, and use it well – The Shawshank Redemption comes to mind." True. Very true. However, professional screenwriters, Frank Darabont included, who have twenty solid scripts under their belt, learn to tell stories without the voice over cheat first, and then when they've become good at the craft of screenwriting, they sometimes (and I stress “sometimes”) use Voice Over for effect. But it should never be the other way around.
Voice Over quite simply is TOO EASY! You must find ways to move your story forward or get additional backstory out through your characters, but it’s not enough to simply have them talking to each other. Use conflict! Use humor! An argument or a joke is one of the best ways to find out about the past.
Think of your own life – when an argument becomes a verbal fight, what it usually disintegrates into is a bunch of finger pointing of what occurred in the past. That's the key. And as the verbal throw-down continues, more backstory is dug up, and before we know it, what starts out being an argument about a husband not cleaning out the dishwasher, ends up being about infidelity and the fact that he cheated on his wife ten years ago with her sister, and she still can't forgive him.