The big screen (not to mention the small screen) is not only a screen that presents the world, but it is also a screen that obscures the world. Below is a list of some prominent big-screen conventions that we need to see past to see the world rightly.
Bop on the head
It's a convenient way to get someone out of the way without killing him. By convention, the person comes to without any lasting damage. In reality, being knocked unconscious is a serious injuring, possibly accompanied by a concussion.
No helmet
The good don't wear helmets, at least ones that would obscure their faces. For example, in Lone Survivor (2013), the heroes repeatedly throw themselves down the side of a mountain to escape the Taliban forces pursuing them. Somehow their heads sustain no injuries, despite the lack of protective headgear. Why? Movies are a visual medium, and the audience has to be able to tell the protagonists apart, and seeing their faces also helps us to identify with them. What's also interesting is how often the bad guys are anonymized by wearing helmets, e.g., the Star Wars stormtroopers. That way we don't mind their being killed.
Sleeping around
Many modern romantic comedies and situation comedies have characters sleeping around. But no one gets pregnant or contracts a venereal disease. I suppose the reason is that if something like that happened, the movie would take on a deeper moral dimension and no longer be a (light) comedy. Of course, in a slasher film, the convention is exactly the opposite: the teen couple that transgresses this moral boundary is usually the first to suffer at the hands of the supernatural antagonist.
Anachronistic sexual morals
The characters in relationships portrayed as taking place in a previous age often hold assumptions about the nature of sexual intercourse that are manifestly characteristic of the modern, anything-goes age. For example, in Sky Captain and the World of Tomorrow (2004), the action takes place in 1939, but the characters in the love triangle at its center talk as if casual relations were nothing remarkable. The Amazon series Man in the High Castle (2015–) takes place in an alternative 1962, but two central characters, Frank and Juliana, live together unmarried, with no one around them making any special note of the fact. The assumption behind these anachronisms is that the upside-down situation that has prevailed since the advent of the Pill is how things have always been. At best, this is shorthand for allowing us moderns to identify with the characters of the past. At worst, it's a kind of cultural imperialism: the present conquering the past, notionally if not in fact, in order to justify itself.
Significant details
Any detail shown is significant to the story. In part, this is just a limitation of story-telling: there's only limited time to convey the action and make it understandable to the audience. But it has the potential to mislead. What's significant to our lives very often we only realize in retrospect, whereas any given detail of our day-to-day lives (e.g., an unexpected coincidence), is unlikely to have a tremendous significance to our life story.
The End is The End
Another story-telling convention. Stories are finite, they must end. But the end carries with it the implication that the character's state at the end is what prevails from that time on, "ever after." In reality, the only real end is death and the end of a story is the beginning of another story—or rather another point in the continuum of life. Episodes of The Twilight Zone have significant endings in this sense, though often the end conveys a notion of Justice that echoes that Final Judgement we all must face.
What are some other conventions? I'd be interested to hear in the comments.
