I spent a third of a semester in my film genres class on Westerns, and one of the first ones our class saw was High Noon. My class had an interesting reaction to that film. There general sentiment was: "This was totally lame. Why did Gary Cooper have to go out into the street? Why didn't he just climb on a rooftop and shoot them from above like a sniper?" Our film professor who had noted at the start of the semester that he had seen easily over 100 Westerns, was somewhat baffled at this assertion. He had trouble explaining it but he just said, "That just wouldn't happen in a Western." It's not what a Western hero does. And that's what we call a film convention. Something we're used to seeing in a certain genre to the point where audiences expect it and filmmakers use it as a language.
I was watching a program of the history channel this afternoon on sharpshooters of the Old West and most historians, apparently, would've sided with my class. Historians said that there might have only been one or two shootouts in the history of the West that happened like High Noon. Most of the time, people shot each other in the back or in their sleep, which would have sent my old film professor into shock that there was little sense of an honorable way to shoot someone back then. The quickest and most efficient way to lodge a bullet into your enemy's head seemed to be the route that was more often taken. It was the pulp fiction novelists who started this tradition of the pulp fiction hero and sold that image back to the folks back East, so that convention has started for longer than Westerns.
Later in the semester, we saw Sam "Bloody Sam" Peckinpaugh's The Wild Bunch which was considered to be later in the evolution of the genre. The film was just as compelling a story as Stagecoach or High Noon, but it was also very bloody and gory. One of the students delivering a report on the film said that one of the themes of the movie was "The indiscriminate nature of the gun." He noted that the film stressed the myth that a bullet would never hit a woman or a child when someone fired it. And it's true: Guns do tend to defy the laws of physics by discriminating against their target. It seems that whenever a sharpshooter aims at James Bond, he has a rare miss. There are whole armies firing at James Bond and he runs fast and manages to emerge unscathed but logically, one of those bullets will hit the guy. The message from these movies is that a true hero won't get shot and that's potentially dangerous to a world.
As the genre has evolved, it seems like film directors like Clint Eastwood, Sam Peckinpaugh, and Martin Scorsesee in the urban crime genre have dutifully attacked the myth and that's where their brilliance comes from. Eastwood's Unforgiven is self-referencing as he inserts a pulp writer (played by Saul Rubineck) directly into his movie of the exact same kind that romanticized the West and those conventions in the first place and through his eyes, we see myths being made in the process.
Scorsesee really has built a reputation for attacking the romanticism of violence head-on. In Goodfellas, he starts out showing the romanticism of the mob life but slowly disintegrates it throughout the film. In The Departed, however, he really targets that convention about the "indiscriminate nature of the gun" I was thinking of.
You go through the movie thinking that Jack Nicholson is one of those invincible larger-than-life heroes who partially because he has been alive so long and partially because of those conventions you think that no matter how many bullets go whizzing by in a shootout, he will emerge unscathed. But, no, just like that, in the most anticlimatic way, he is shot by Matt Damon in something that isn't thought out particularly well. You get the feeling that if Matt Damon were thinking through his actions, he wouldn't have shot the guy who he'd been devoting all his time to risking his career to protect. But that's just the point: to shoot a gun, you don't have to think about it very long. All you have to do is just apply one inch of pressure and in a quarter of a second, massive consequences can happen.
The same thing happens to Leonardo DiCaprio. Because he's the hero and we're conditioned to believe, he'll get his happy ending, we're absolutely in shock because it all happens so quickly. But that's really how guns work.