I'm at a loss here. I'd like to describe a moment where one of my characters just loses it, no warning, because of a culmination of things that had happened to them over the course of the story. It needs to be sudden, like glass shattering sudden.
You can do something like... Without warning, his senses left him. It felt as if another being had taken control of his body, his emotions. All he felt was pure, blind, seething rage; he saw himself charging Mike with a loud roar, saw his hand turn into a fist before smashing into Mike's cheek. Any emotions, any restrain was now gone. Only one word existed in every fiber of his being: Make Mike Suffer. Oh he would. He would make Mike suffer for every last bit of wrong he had done. Now straddling over Mike, he grabbed his throat with one hand to keep him pinned and let his fist fall again. With every blow, he felt a wash of euphoria come over him. This was for calling my wife a bitch, he thought. This was for pushing my friend over because he was in the way. This was for lying, cheating, stealing and generally making my life a living hell! He heard Mike cry out for mercy, saw the anguish in Mike's face through the blood. Good. Begging. I like that. Let's hear some more. Suddenly, he felt hands seizing his shoulders, his arms, pulling him off of Mike. That's just an example, though. You can make the person do other things. Generally, I think it's when the person looses all sense of reason and it's like something else had taken over their body, their thoughts. They might see what they're doing, but they can't stop themselves from doing it.
You can show it through his/her actions, just say what they do, show the fact that they've lost it, and if there's no build up then it'll be a shock to the reader. Does the character start to break things, start shouting, crying? Maybe they try to kill someone.
Link has the right idea--it's often shown as sudden physical action, e.g. leaping up/smashing a fist on a table, then maybe the character finds him/herself doing something like fighting or tearing off down the street, with a few seconds memory hiatus in between.
Hmm...It's sort of like they suddenly turn dangerously quiet, with a shadowed face, head tilted to the side, looking forward with a dead stare sort of insanity. it'd be easy to do if it were a visual medium.
If you are doing it from the observer's POV, it's just as easy to do it in writing. You describe it. If you want an in-depth feeling of what is going through the head of the character who is losing it, it is waaay easier to do it in writing than in a visual medium. Anyway, visual is only as good as the actor makes it.
Yeah, that's 3 words Yes, exactly. Your example is a bit over-thought, because you're explaining his motivation as you go. I prefer getting out of the characters head a little and just showing what they do from an outsider's perspective - otherwise you risk psychoanalysing and justifying it too much in the heat of the moment. Most people don't have that internal monologue going when they snap, they just.... abandon reason and act. There is nothing but rage, which is the absence of reason. I've felt this on numerous occasions, and in those moments I could actually kill someone - I really do understand how people can do it when they just lose control like that. Because you're not thinking at all, you're just doing.
If he's snapping suddenly, I'd have a sudden action totally catch the reader off guard. (ie he throws something across the room without any warning, although this will likely be seen as cliche. lol) Although, I'd also foreshadow it a little bit without making it obvious.
It isn't so much "show, don't tell" as "don't bludgeon the reader with the obvious." If the character is shielding his face against wind-whipped snow, you don't need to add words like cold, frigid, icy to the description. Keep your narration lean. Don't insult the reader's intelligence with redundant adjectives, and especially adverbs.
I think the most powerful "breakdowns" are not the ones that get loud and in your face, but the ones that lose all emotion. Take a character that is super overwhelmed with emotion, is freaking out, crying and everything, and then all of a sudden gets quiet. Stands up and stares blankly. Pulls out a gun and shoots himself in the head. Not the best example, and you might not want to kill your character, but I just wanted to quickly illustrate the point.
I think action first; write something pretty normal, or have him talking to another character in a relatively normal way, and then BOOM! - put some kind of action in (like a punch, or a screaming fit or something) that will shock the readers then explain afterwards; 'I froze, unable to believe what I'd just done. It was like something was unravelling inside of me after too many years of being tied and twisted up'.
