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.

  The last week or so has been tiring - especially for an introvert. A people-y writers conference (great, but tiring), christmas parties etc. So at one point I had to just stop and listen to my body and say 'Nope, staying home today.' 
In martial arts, learning to control and listen to your body, then adapt your skills to suit is crucial. A dojo might teach a technique the same way to everyone, but that doesn't mean it will work for you the way it works for the guy next to you. Trying to do it exactly the same will, in fact, lead to frustration and poor skills.
     Every person is unique in the way they move, the way their body works (or doesn't), their speed, muscularity, flexibility etc. My husband has extremely inflexible joints but mine are hyper-flexible. Our actions to perform techniques are different to compensate.  Things that work easily on him have no effect on me, and visa versa.
     You may have physical limitations that mean the technique needs to be modified to suit your body. Perhaps your knees are dodgy, or you have shoulder injuries. If doing a technique in a certain way physically hurts you (beyond just a twinge), then STOP and find a new way to achieve the result you want without damaging your body.
     Ignoring a small amount of pain is good as it teaches you to push through inconvenience to achieve your goal. Ignoring extreme pain is stupid and will result in permanent damage.
Don't let pain and discomfort or limitations hold you back. Just learn to listen to your body and adapt. One of the best martial artists in my dojo had only one arm. 
     In writing, you'll get handed a lot of 'writing rules'. Show don't Tell. Don't use Filter words. Don't start with a dream sequence. Your Inciting Incident MUST be at 12%. Use only 'said' and 'asked' as dialogue tags. etc etc.
     And many of those (and other) bits of advice are helpful and will improve your writing if you're an emerging author - even if having them pointed out in your writing is painful and distressing. Ignore that pain. It's minor and important to push through. Definitely don't write off your editor and arrogantly dismiss their suggestions for improvement. That would be like ignoring your multi-blackbelt sensei.
BUT...
     Think of the 'rules' as the martial arts techniques. Once you understand them, THEN you have to listen to your own body and mind. Listen to your gut and do what serves your story best. Sometimes Telling is the best way to get from one scene to the next without tediously showing  a long trip across boring deserts. Use a 'wrong way to do it' IF it is vital to the story and with the awareness that it might not work and you might have to experiment with something else.
     And - even more importantly - listen to your body when READING. If you want to learn how to build tension in a story, read a thriller and take note of when you feel the tension twist in your stomach. Then stop and re-read the passage to see how the writer did that. If you find yourself crying, stop and re-read to see why you got so emotional.
     Paying attention to your body's reactions when you read will help you become a better writer. And once you understand the 'rules' you'll be able to modify the techniques to suit your story and be deliberate about how you guide your readers' emotions.