How do you introduce your reader to your fantasy or science fiction world? You have a protagonist that’s seeing it for the first time, of course. They can point out the fascinating stonework on that never-conquered tower, or the intricate knobs on that FTL drive. But the thing about exploring exotic locales is that the people there are foreign. And when you’re far enough away from home, the little differences can shock and alarm you.
When I was in college, I had roommates from Argentina whose cousin came to visit. They warned me that in Argentina, people kiss on the cheeks when they greet. I considered myself a very worldly twenty years old, and thought it wouldn’t faze me at all. Then he showed up, and he was really cute, and instead of acting casual, I blushed furiously and froze. So much for sangfroid.
There’s a difference between knowing what the customs of another culture are, and actually being familiar enough with them, through experience, that they are second nature. Newcomers to Japan remember that the Japanese bow, but it takes a few months before they find themselves bowing during telephone conversations.
Cultures decide what’s acceptable levels of intimacy from strangers, and what aren’t. Even subcultures have different rules. In my mom’s cult, everyone hugs everyone else for about twice as long as you think they ought. By now I’m used to it, but it took a few awkward embraces with strangers until I knew what to expect.
I used this in some of my novels. I have these people called the Indel, and they have a fairly cohesive culture. One of the aspects of their culture is that they stand very close to one another when speaking. They bow to greet, so they have to bend the knees more than the waist so that they don’t bonk heads. This is all well and good when they’re with each other, but then they meet my American protagonist. As my Indel steps forward to get at a convenient speaking distance, she steps back, and back, and back, until she hits a wall. Then she feels uncomfortable, and he feels like she’s being standoffish. Yay! Conflict! I love making my characters squirm.
Consider also notions of privacy. If your character was startled in the bath by a stranger, would she cover her breasts, her genitals, or her face? Or something else? What if female breasts were considered perfectly suitable for public display, but a woman’s hands were not? How would they castigate the gloveless woman? I have another race in which men and women both go bare-chested. My protag, again, an American woman, does not want to run around topless, so she wears her jacket. Their theory as to why she does this? That her culture is extremely patriarchal and forces women to hide themselves.
The other aspect of personal space we don’t often consider is the notion of public sphere and private sphere. For some, if you eat with them, you are in their private sphere. “You drink with me, you are my friend.” For others, you cannot be in the private sphere until you are family. In the novel Samarkand, by Amin Maalouf, there’s a scene where a foreigner is seeking refuge with a Persian woman, and she can’t let him live in her house unless he’s part of her family. So she suckles him at her breast, and then he’s her ‘son’ and can stay there. I take the incident with a grain of salt for accuracy, but it really fired my imagination.
There’s no aspect of culture so strange that it doesn’t or hasn’t existed somewhere.
Clarion Blog 
This is fascinating – and so true. There’s also things like asking personal questions (a polite expression of interest and friendliness in some cultures, intrusive nosiness in others); talking over other people (showing involvement and excitement in the conversation – or rude disregard for the other speaker); or even the now-famous Indian head-shake (meaning yes) that looks like no in US/UK body language. The challenge in writing, I find, is using something like that lightly and not obtrusively.