Let's face it, people are both incomprehensible and fairly predictable. It's a maddening dichotomy that causes much angst. We (unconsciously) expect people to think and behave as we would, then are astonished, blindsided, and often angry when they don't.
One of the most important things you can have as a martial artist and as a writer is the ability to put yourself into someone else's head. (Not literally - unless, perhaps, you're a horror writer doing hands-on research. Ew!.) To understand WHY they chose to think and behave a certain way.
As a martial artist, you develop the habit of watching people. Scanning them up and down. Analysing their behaviour and predicting what they might do next. It's a useful habit even if they aren't a threat. People usually give away their next action in their body language and facial expression.
It's quite hard NOT to judge and anticipate behaviour based on how people look, speak and act. As a troupe animal, humans rely on 'reading' their companions' moods and reactions in order to adjust their own behaviour to 'fit in' with the troupe (and therefore survive).
A good martial art just trains you how to read people for a slightly different purpose - to deliberately neutralise threats before they emerge.
As a writer, it's vital to learn to understand people. Because the more you learn to anticipate how people will behave in any given situation, the more you'll be able to write people who aren't YOU on the page. If you would never dream of cutting someone off in traffic or working 80 hour weeks - because you're just a super-chill, easy-going person - then you'll have difficulty writing an aggressive workaholic.
You don't have to BE the characters you write, you just have to deliberately study how other people think and behave.
Which results in the ability to write a wider variety of characters that act in ways consistent to their own, unique personalities.
And, as an awesome side-effect, it might even help you be more accepting and tolerant of people you love but perhaps sometimes get on your nerves. Or people you've just never understood before.
Lets face it, any extra tolerance and acceptance is a great thing in today's world.
So go out and dig up a few books on personality types, on psychology, on human behaviour. Learn to understand people so you can write 'real' characters.
Maybe even help neutralise threats by making people more openminded and accepting.
I live in Australia - which tells you I have a sense of humour. We're a self-deprecating people, we Aussies. My aim is to, one day, vanish in a blinding flash of enlightenment. In the mean time, I'm doing my best to learn as many