Like a good joke it’s all in the build up, and like a good joke the act itself, or the punch line, has to be short and comprehensive.
Any act of violence has to be preceded by the perpetrations of a sufficiently odious antagonist. This may sound like a moot point but is something that is often overlooked. It is advised that a reason as to why a protagonist resorts to violence rather than find a peaceable way be made clear to the reader. If you are writing a character who goes around dispatching people who have only infringed on him or her negatively to a relatively mild degree will probably leave the reader finding it difficult to relate to the character.
In real life serious acts of violence happen very quickly with little warning. However in prose we have to slow the whole thing down.
The violent scene, say between two parties, should go a little like this:
Our hero/heroine finds themselves squaring off to their suitably leering nemesis, or one of the nemesis’s henchmen.
The build up
By this point in the story the reader should have been made aware of the withering torrents of abuse bestowed on our hero/heroine by their nemesis and his/her associates. Plus the virtues of the protagonist and be in sufficient knowledge that the antagonist is a bastard that needs to get done. This is a good point to start the feelings of rage and fury, in as much description (both physical and psychological) as possible. Maybe throw in a few of the protagonist’s memories of both their present grievances and perhaps of past similar hard times for good measure. Into the mix add the characters feelings regarding justice and the unjust actions of the antagonists et al, to give it that old school feel. This should be punctuated at the end of this phase by the protagonist resolve that ‘they ain’t having it’.
The wait for it
Now your character is nice and primed to loose control of their sensibilities and give’em some ‘what for’. This part should describe the protagonist weighing their opponent up, and deciding on a cause of action based on the circumstances. Like a sports commentary this will take into account strengths, weaknesses and any injuries or aspects that may impact on any of the parties’ performance at the time of the scene.
The Punch line
Boom. The Clash. Ideally, you’re not looking at doing some kind of Jerry Bruckheimer chestnut where everyone is adhering to some kind of rules and it looks all fancy. Keep it real. Punching, kicking as well as biting, gauging, head buts, fish hooking, stamping, hair pulling, scratching, crotch shots and working on the joints should be the order of the day here. First rule of the actual fight scene: there are no rules.
Try and get your facts right
As with all things in writing a little research will go along way. Such as a human skull will often cave in, as if soft, like an Easter egg, rather than crack apart like a nutshell.
If you are in a situation were the protagonist is up against a larger party the best way to approach this is to have one or two of the henchmen being really handy, and maybe even secondary characters, or even featuring the primary antagonist him or herself, while the majority of them are mugs. If, however, the protagonist is up against a crack team of nutters, in the interest of keeping it real it is best to have the character receiving a great deal of injury and even getting soundly beaten. Have the character ruminate over this and maybe dispatch the perpetrators when they’re in more manageable portions.
There are many day-to-day things that will help put you in the mood for writing violence. Things like dealing with bureaucrats, bank managers, other authors, an idiot in a pub or printers for example. Reading the newspaper and beholding the latest sleazy escapades of a politician can do the trick too, as well as trying to derive sense from any given authoritarian. In brief; get mad, write violence.