When

The timing of your fic is very important. If your fic is supposed to be about Valentine's Day, it shouldn't be happening on February 29th unless there's some sort of explanation as to a) why the characters are celebrating it on the 29th or b) how Valentine's Day got moved to being every four years and the same day as Leap Day.

Aside from the obvious, though, when your fic happens can have a huge impact on how your story plays out.

General Things

• If your fic spans the course of more than a day or two, it's probably a good idea to create a timeline for the major events of your fic. That way you can avoid chronological inconsistencies.
• Keep in mind the amount of time it takes for certain things to happen.
• The year: In a present day fic, it determines your idol's age. Also, if you're writing a fic set in about 2009, the iPhone 4S had not been released yet, so your idol shouldn't be carrying one.
• The academic calendar: the South Korean school year starts in early March and ends in mid-February. Look up when students get vacations and for what holidays/reasons.
• Seasons: if it's been over three months, the season should have changed.
• Months: if your OC suspects she's pregnant, then missing her period would be important, and also, if she really is pregnant, how many months have passed will definitely matter because that determines how much the baby has developed and what sorts of "fun things" she'll be experiencing.
• Holidays and birthdays: these tend to be things that important events and scenes of fics are based around, so you want to keep track of those.

In the Real World

Let's first assume that your story is happening in the real world, or something close enough to the real world that the differences are negligible.

• If your fic happens in the past, then you need to do your research on the era that you're using. Certain things that we take for granted in contemporary times were nonexistent or quite different in the old days. It can affect everything from the way people dress to what they drink to how they speak to certain inventions to meanings of words (which are sometimes reversed or completely altered over time). A woman suing in court or owning property, for example, wouldn't have happened several centuries ago.
• If your fic happens in the future, then it's easier for you to call the shots on what exists and is possible and what isn't. However, you still have to keep it believable (time travel in 2015 is not likely) and try your best not to just totally rewrite history.
• Even if your fic happens in modern times, you still need consider all of the things I listed earlier in the general tips.

Idol Life Fics

Honestly, any idol fic is going to have to bend the truth on matters of time because it's pretty much impossible for idols to have much a life outside of their work. However, you should definitely still keep it as realistic as possible. You need to consider:

• Idol Histories: what they've promoted, what shows they've been on, what scandals have happened to them, etc. If you want to keep timing accurate, you'll have to factor in all of those things. If you are doing alternate history or tweaking the timeline, then you still need to apply the logic of what idols go through--two to three promotions per year, concerts, etc.
• Idol Schedules:  They shouldn't have only four hours of work time and then twelve hours of time to themselves. Even during the times when they're not promoting, they're still practicing, preparing for upcoming promotions and concerts, appearing on variety shows, acting, doing other side activities, etc.

In an Alternate Universe

Don't get cocky and think that if you make up a new world, you can do whatever you want and time doesn't matter.

In addition to the general things, think about:

• The history of your AU: History matters, even in other universes. It would make a huge difference if your character lives in an era in which all of the people live as one united political entity vs. a fragmented bunch of nations or kingdoms that are at war vs. two hundred years after international/interkingdom peace was achieved.
• The era that your characters live in: Does it resemble or is it based off an era in the real world? Again, it's good to do research about that era. After all, what's better than actual history for creating realism?
• Units of time: We take for granted things like millennia, centuries, decades, years, months, weeks, days, hours, minutes, seconds. If your alternate universe is on some other planet orbiting another star, the length of a year will be different because it will take a different amount of time to go around the star once. Base ten isn't the only possible way to group years. Base sixty isn't the only way to subdivide increments of time smaller than a day. Most people do not change these things because it makes things more complicated and even confusing, but it's something you could potentially change and should think about.

Where

How Far and Wide

Your story may involve just a small area of land, such as a neighborhood, but it could also venture into other parts of the town/city, as well as other counties, states/provinces, countries/kingdoms, continents, planets, star systems, galaxies, universes, etc. The broader the scope of your setting, the more the interrelationships between each of these units and their neighbors play a part in shaping the lives of the characters and the plot of your story. People don't just live in a vacuum (and I don't mean the cleaning device).

It takes time to get from place to place. The passage of time and any events that happen while traveling from one place to another are things you need to think about. Not all places are the same. Your character may feel more or less comfortable in one place versus another, especially if they're crossing cultural boundaries. Languages and laws will probably be among the significant differences aside from the more everyday or broad cultural customs. The discomfort they feel can go as far as things like claustrophobia (fear of tight spaces) or agoraphobia (fear of open spaces) or culture shock (which results in varying degrees of physical, mental, and psychological distress).

Geography

Sounds like school, doesn't it? Well, there's a reason you learn it, and the lessons you gain from it are applicable to fan fiction.

Geography can affect:

• Politics: conflicts arising from regional differences and disagreements.
• Economics: how and what and with whom people trade, what natural resources are available.
• History: way back in the day, cultures were heavily place-dependent once most of our human ancestors settled down and began that thing called agriculture.
• Architecture: where you build dwellings, what you build your dwellings out of, what they need to be reinforced against or withstand.
• Social values: different aspects of the physical world can gain certain connotations and associations; skills and abilities that play a part in adapting to and surviving the surroundings might gain emphasis.

Landforms and Bodies of Water

They affect how cultures develop and also how multiple cultures interact--or don't interact--with one another, so spend some time thinking about the lay of the land in your AU. Different countries may go to war over a piece of land; a water supply may be a source of conflict. If your country is an island, do they have trade relations with other countries, or are they isolationists? If your country/kingdom is landlocked, are its neighbors eating away at its borders or using that fact to take advantage of it? And so on. If it's on another planet, do they have interplanetary travel and communication?

Weather, Climate, and Seasons

These factors affect:

• How people dress: light clothing vs. heavier clothing, color of clothing, sleeves/lack of.
• Holidays: celebrating things like solstice, mid-winter, mid-summer, end of winter, end of monsoon season.
• Religions: gods or spirits that control and/or embody these forces.
• Amount of outdoor activity and what types of activities that people engage in.

Biomes and Ecosystems

Ever thought about what kind of trees exist in your setting? What animals? For AU fics in particular, these will be important, or at least things to think about for world-building. Although the flora and fauna seem trivial, they can come into the spotlight if they have have any combination of religious, spiritual, cultural, economic, scientific, medical, or magical significance (not so trivial after all, eh?). What kinds of fruits and vegetables can be grown? What livestock can be raised? This affects the economy of the region.

The Importance of Culture

Along with "when," the "where" of your fic determines the culture that your character live in. Culture covers a wide variety of things, from language and gestures, to music and clothing, to laws and morals. People within a culture will often take those things for granted. As an American, I take for granted: my right to free expression, the individualistic spirit, the value on time as something to be "spent" on productive things, nodding means "yes" (dunno if some of you know this, but yeah, nodding means "no" in certain cultures; there's nothing inherent to the gesture as signifying agreement; it's an assigned meaning), having a personal bubble and physical comfort zone that extends two feet or more around me, etc.

When writing fanfics, a lot of people completely overlook the importance of culture and write their stories as if they're taking place in their own country/culture. I will admit I've been guilty of this in my own writing. We as people often don't realize or think about the fact that people in other places operate in a different way than we do and that sometimes these differences are very extreme. That's what I mean by taking things for granted. We assume a lot of stuff that we really shouldn't. In real life, if you're in another country and culture, these assumptions, albeit innocent without ill intent, can lead to serious problems in communication, breaches in personal comfort, and even conflicts with the law. In the realm of writing, they create a problem in terms of accuracy and realism and therefore believability.

Two Axes of Culture

Whether you're creating your own country/culture or using an existing one for your setting, think about the following:

• Cultures can be considered collectivistic or individualistic. Collectivistic cultures place society as the center. Image is important and should be maintained at all costs. Connections are advantageous and even supersede individual skills in priority when it comes to getting places in life. Conformity is a strong force; you generally don't want to stand out. Individualistic cultures place the self as the center. Getting ahead, improving on your circumstances, doing what you want to do--all of these are recognized as positive and rewarded. Standing out is a good thing.
• Cultures can also be considered high-context or low-context. From Wikipedia: "It refers to a culture's tendency to use high context messages over low context messages in routine communication. This choice of communication styles translates into a culture that will cater to in-groups, an in-group being a group that has similar experiences and expectations, from which inferences are drawn. In a high context culture, many things are left unsaid, letting the culture explain. Words and word choice become very important in higher context communication, since a few words can communicate a complex message very effectively to an in-group (but less effectively outside that group), while in a lower context culture, the communicator needs to be much more explicit and the value of a single word is less important."

These two types of classifications are not either-or. They span spectrums, with some cultures being more toward one extreme than another. The United States has a very individualistic and low-context culture compared to most other countries. South Korea, on the other hand, is relatively collectivistic and high-context. Keep that in mind as you're writing.

In South Korea

Most kpop fans have some degree of familiarity with the following cultural traits and readily include them in their fics:

• Basic greetings and expressions (annyeonghaseyo, kamsahamnida, omona, aigoo, etc.)
• Honorifics (oppa, hyung, appa, eonni, etc.)
• Last names come before first names
• Bowing as a form of greeting someone
• The value on age/seniority (though not necessarily the degree to which it is valued)

Personally, the first bullet kind of bothers me. To me, overuse of those phrases makes the author seem like a bit of a wannabe who's trying to compensate for lack of more complex and detailed knowledge of the culture. Also, pretty much all of those phrases have perfectly good English equivalents. I suggest that you avoid using Korean words or phrases unless there isn't an accurate equivalent or translation in English. Things like food names, traditional Korean music terms, etc. are okay to leave in Romanized Korean. Sentences like "annyeonghaseyo, ____imnida," on the other hand, should be eliminated and replaced with English. Also, do not use Korean phrases unless you are absolutely certain you're using them properly. Words and phrases often have multiple meanings and have different implications in different contexts, so for all you know, a phrase you use could be completely nonsensical in the context of your story.

Here are some other general South Korean cultural traits:

• Social conservatism: Men and women have more rigid gender roles; things of a ual nature are more heavily censored in public, whether in the form of self-censorship (people don't talk about them) or government censorship (they get banned or labeled with ratings); homophobia, regardless of degree (from disgust to disapproval to misunderstanding of homouality), is widespread.
• Religious apathy: A decent number of Koreans are Christians (about 30%), most are either Buddhist (about 23%) or Atheist/Agnostic (about 46%). Confucianism, if you want to count it as a religion, has influenced values in the culture, but most don't actually call themselves Confucians.
• Emphasis on education: South Korean students spend far more time on school and academics than students of other cultures. If you don't realize the amount of time it eats up in their lives, take a look at this. Although it might be more of an extreme case (as the article points out), it is very much true that Korean students don't simply have hours and hours to spare after school during the weekdays the way students in other countries might.

Note: Do not trust manhwa, dramas, and movies as accurate depictions of life in Korea. As we should all know, the media often distorts reality. I, for one, can testify that, contrary to all the movies and TV shows, high school and college in the U.S. are not at all centered on partying unless you make it your life's focus, and even then, the frequency and availability of parties in high school are very much limited. Most people have a pretty even balance of school and social lives.

A resource I found for detailed descriptions of the daily life of a Korean teen is this. It's from the early 2000s, so it's a little bit obsolete/outdated, but I think most of it should still be valid and applicable. Pop culture may have changed drastically since 2001, but the other aspects of Korean culture most likely have not. In addition to reading that, I suggest you do some more research on Korean culture. 

Seoul

Most fics that occur in Korea take place in Seoul, but honestly, I have not really come across many fics that demonstrate any actual knowledge of Seoul's layout and its districts and their associated traits. (I am also guilty of this.) I think it would benefit everyone to learn a bit about the different parts of Seoul. It is not one giant, even lump of city. It has a varying "landscape" as well.

Again, bending the truth a on cultural things is almost inevitable due to the nature of the fics most people write (especially male-male or female-female romance fics). However, limit that tweaking and distorting to very specific cultural traits and try to keep the rest as true to reality as possible. Otherwise, you're not really writing them in their culture, and you might as well create an AU that kind of seems like Korea but isn't.

In Non-Korean Real-World Countries/Cultures

Unless you are writing your fic in your own country/culture, once again, you will have to do some research. Look up those topics that I discussed in the South Korea section. I believe most fics that are in other countries are vacation visits rather than alternate realities where they live in other countries. Thus, you should probably look up those touristy tidbits such as famous foods and landmarks, money exchange rates, etc.

In an Alternate Universe

You have a lot more freedom, that's for certain. However, as I said with the section on "when," it's a good idea to base places in your world at least partially off of places in the real world. Just looking at all of the things I've listed out in this chapter, which don't even cover every last aspect of culture and geography, you can tell that completely constructing a brand new world and culture can be a lot of work. That's why you can cheat draw from reality.