One of the most basic (and most important precepts) about film making is that the story most be told entirely by the visuals. Meaning that you should be able to watch a movie with the sound turned off and still get everything that's happening on screen and understand the whole film. The great film makers of the past knew this and that's what made their films so great.
Like any basic and irrefutable truth, I find that people often reject it and question why it's important. And it's always hard to articulate why it's important because (to me) the answer is obvious.
People will say "well, we're making a movie with sound. We have dialogue at our disposal. We can just use words to convey our meaning".
What I would answer is that whoever said "A picture is worth a thousand words" had it right.
The problem with words are that words can be cheap and disposable. We don't always pay them a lot of heed. When you're talking to people in your everyday life, do you always catch every single word they say? No, we're frequently distracted by our own thoughts and we don't catch 100% of what people say. The same is true of movies, and if the audience misses a bit of dialogue because the person behind them is coughing or opening a noisy bag of Skittles, you don't want them to be lost for the rest of the movie.
Also words aren't always the truth. When we listen to people talk in real life we run it through our filter, trying to figure out what is the underlying truth about the words they are sharing with us. We know that sometimes people lie to us, and sometimes they are telling the truth from their point of view but they may not know the whole story. There are a million reasons why their words may not be truthful - deliberately and otherwise - but the point is that we never take what people say as the absolute truth because we know it's not.
On the other hand, visuals don't lie. With a picture what you see is what you get.
Sometimes film makers will lie to us with their visuals, based on showing us a false image, or leaving out a key image that makes us interpret the ones before and after it differently, and then revealing the truth to us later with an additional image. And that's always more powerful than just finding out someone used false words with us.
We can remember an image that we saw with perfect clarity for the rest of our lives. But how often do we remember exactly what someone said?
So when I try to explain this concept to people I just say that a story that's told through images has a deep, visceral impact on the psyche of the people watching it because images operate on a much deeper level than words. Obviously, the best movies use both in concert to convey their story. But images are always, always, always more powerful and supercede what the dialogue is telling us.
The biggest reason why people seem to reject the wisdom of this concept is that - like most basic concepts - it's really hard to do. It requires discipline, knowledge and hard work to tell a story through images. And in my experience people will (ironically) go to great lengths to avoid having to use discipline, knowledge and hard work.
Usually if I've gotten this far in my explanation they're still not convinced. So maybe the words of author David Mamet will convince you, instead. From "Bambi vs. Godzilla", page 152:
"The perfect film is the silent film, just as the perfect sequence is the silent sequence. Dialogue is inferior to picture in telling a film story. A picture, first, as we know, is worth a thousand words; the juxtaposition of pictures is geometrically more effective. If a director or writer wants to find out if a scene works, he may remove the dialogue and see if he can still communicate the idea to the audience.
Ancient theological wisdom put it thus: 'Preach Christ constantly - use words if you must.'"
To be honest I'm not really a fan of Mamet's films, but he's written several great books on film and acting that I definitely recommend.