"Mmmm, I'm full off of all your collective disappointment."
The Slow Clap
One of the most ubiquitous clichés in film, the slow clap shows up so often and across so many genres of movies that entire compilations have been created in its honor.
Its power lies in the slow clap's ability to sway the opinion of an entire crowd, not with words or ideas, but with the sound of two pieces of flesh slamming against each other. The slow clap generally follows the protagonist having an, "I'm just being honest for a second" moment in front of an audience. While everyone stares on horrified (everyone in movies presumably hates honesty), one person begins to clap. Somehow, that sound alone is enough to convince everyone else that, oh yeah, they also enjoyed that crazy bullshit that just happened. Gradually it swells into a massive applause as the opinion of everyone in attendance shifts 180 degrees simultaneously. The slow clap as a concept seems absurd; an entire crowd of people convinced they like something just by hearing one person clap is insulting to human intelligence. Oh, and it turns out it happens to crowds all the time.
The psychology behind it:
Even the most opinionated and judgmental people in the world are susceptible to the power of group mentality. Arguably the most prominent researcher in crowd psychology, Gustave Le Bon suggested that when people come together in crowds, they start to identify with the group at large instead of as an individual. The opinions of the crowd become the opinions of each person like a mass hypnosis.
"A phenomenon of which it is easy to establish the presence but which it is not easy to explain. It must be classed among those phenomena of a hypnotic order. In a group every sentiment and act is contagious, and contagious to such a degree that an individual readily sacrifices his personal interest to the collective interest. This is an aptitude very contrary to his nature, and of which man is scarcely capable except when he makes part of a group."
In other words, every minute change in the crowd has a ripple effect on the collective conscious and can determine the thoughts and opinions of the rest of the group. While this sometimes manifests itself in riots or acts of violence, the slow clap is a decidedly more positive symptom of mob mentality. A single person clapping in a crowd can immediately trigger the same response from everyone else because they are subconsciously serving the interest of the group, until finally, everyone is in unified agreement that they love whatever just happened.
Even if they can't remember what the hell it was.
Getting Moral in the Mirror
Sometimes in the middle of a movie a protagonist has to lock himself in a bathroom, splash a little water on his face and have a quick one-on-one with himself in the mirror. It usually happens when he's faced with a ethical decision; Vincent Vega talks himself out of sleeping with Mia Wallace in Pulp Fiction, and Smeagol cures his own schizophrenia and decides not to kill Sam and Frodo just by talking to his own reflection in Lord of the Rings. Protagonists consistently use a mirror as a moral compass, the only exception being women, who for some reason make really terrible decisions after seeing their own reflections.
"C'mon, what's the worst that a couple of business men would ask me to do?"
In reality, hashing things out with a reflection would seem silly if not a little crazy. Most people haven't talked to themselves in a mirror since they were children. Yet, it turns out that more people probably ought to face themselves when dealing with a decision of principle, because it actually works.
The psychology behind it:
Studies have been conducted on both children and adults to determine how confrontation with our own reflections affects our morality. One self reflection study conducted on Halloween gave kids the opportunity to grab candy out of unsupervised bowls. The bowls were exactly the same at separate houses on the same street; the only difference was that one bowl had a mirror behind it. As a result, the children were significantly less likely to steal candy when they had to look at their own reflections even though they were wearing costumes. A similar test was also done on adults at a news stand who paid for newspapers on the honor system. The results were nearly identical; we are all more likely to act morally while confronted with our own faces.
"I slept with Chris before you were dating, and once during. Break a leg tonight!"
So while the mirror pep-talk provides a nice cinematic analogy; the protagonist faces his physical reflection and through it can re-calibrate his personality, our minds really do work that way. We as a species just make better decisions when we have to look ourselves in the eye as we do it. So if you know someone who got rid of every mirror in his house for the sake of feng shui, then congratulations, you finally have a legitimate reason to suspect he may be terrible person.