Around a character with a Button you have to check and double-check that what you are saying doesn't relate in any way to their height, their weight—to their Button in any way. At least they are normal most of the time.
In comparison to the "avoid the Big Red Button" approach with the above, if a character has a Hair-Trigger Temper, conversation is like trying to navigate a mine field with no metal detector. You know the danger's there—in fact, it's probably all over the place—but you have no clue exactly where it is. In addition, the man is not normal until proven otherwise. At best he's tetchy and anxiety-inducing. At worst, he's a surly, dangerous, ticking timebomb. The very act of talking to them might be enough to set them off.
Such a character is often a foil, nemesis or antagonist for the Naive Newcomer. The two will probably get off on the wrong foot from day one, and no matter how hard the rookie tries to be friendly, the character with the Hair-Trigger Temper now has him marked out for punishment. Expect some Filler or comedic sidestory as the unfortunate victim tries to figure out just what topic sets his antagonist off, so that he can avoid it in the future.
Eventually, provided that their I.Q. is higher than that of cheese (less common than you think), it will dawn on the Naive Newcomer that there is no specific Berserk Button. This character will find an insult in any sentence, even if he has to reduce each word to its Latin roots to do so. The point isn't that he's sensitive to some fairly innocuous issue, the point is that he's trying to pick a fight, or to intimidate someone. It's effectively the verbal version of Why Did You Make Me Hit You?—behaving badly while pinning the blame on the person being attacked—although it's more outright aggressive.
Characters with a Hair-Trigger Temper usually have a favorite victim for their "What did you just say?!!" rampage, though they're grumpy to everybody.
While conversation with a character with a Hair-Trigger Temper is risky, trying to joke with one is downright suicidal. If you end up Digging Yourself Deeper, or fall down a Freudian Slippery Slope, you're screwed. The only advantage you might be able to claim is that they're essentially bullies, and if someone tougher or higher ranked than them shouts "shaddup!" very loudly, they probably will. Characters with a Hair-Trigger Temper do not respond well to meeting their match (although they might, as in the movie Goodfellas, take revenge by a sneak-attack). And forget ignoring them—if they have to physically scream in your face to get attention, that's what they'll do. After it's over, you may note to yourself that they're basically overgrown toddlers, but it's difficult to remember that when you're being ranted at in the middle of the street.
The level of violence sparked in these characters differs greatly. For some it's all verbal, but the most violent ones (such as the hothead from Goodfellas played by the former Trope Namer) are a twitch away from shooting someone over nothing.
Unfortunately, people with a Hair-Trigger Temper are Truth in Television. Most people can remember at least one—the Drill Sergeant Nasty or the Sadist Teacher who twisted every word you said into grounds for detention, or a so-called friend who shredded every nerve you had as you tried not to "insult" them. This characteristic is also found on message boards. Since no-one can really project a tone of voice on to a typed message board post, genuine misunderstandings are common enough as it is. However, some boards have members who would start World War III over a comment such as "I like pie." What happens next depends on where the original poster and the objector fall in the Forum Pecking Order - if the person with the Hair-Trigger Temper is a high-ranking member, the newbie is doomed.