Probably one of the most repeated bits of advice to writers in classrooms and workshops is “show don’t tell.” For example, rather than just telling the audience, “Joe was angry” it’s largely considered better writing to show that Joe is angry through his words or actions. “Joe slammed his fist down upon the table,” for example, would be showing us that Joe is angry without directly addressing the reader.
Like with all writing advice, there are exceptions. For example, if you have a character avoiding eye contact with another character, this can mean different things depending on context. In some cases, avoiding eye contact could be seen as a sign that the character is being deceptive. In some cultures however, making eye contact is considered rude behavior, so it could just be a matter of being polite. In some cultures, servants or lower classes are expected to avert their gaze from their superiors. Also, people with autism sometimes find it difficult to make eye contact. If you’re writing fantasy or sci-fi, the avoidance of eye contact could mean something else entirely. Unless you want to make the lack of eye contact a mystery for your readers to discover the cause of later, you probably should tell them what it means.
Of course, even in this case, it’s still possible to convey what the action means without directly telling the reader. Perhaps have a character say something like, “You insolent cur! Avert your gaze when addressing a superior!” There will still be times when telling is necessary, but if you’re able to show as much as possible, your readers will be able to forget they’re reading a story and be able to get more immersed into your world.
It’s of course impossible to explain everything. Any piece of writing assumes the reader has a certain cultural context. Consider who might be reading your book. If you assume your readers know everything you do and not explain anything, your readers will often be lost. If you assume your readers don’t know anything and explain everything from what a car is to how many arms and legs humans generally have, your readers will feel like you’re insulting their intelligence. You can’t please everybody, but you need to strike a balance between explaining too much and explaining too little. So you need to have a particular audience in mind whenever you write something.
I’m currently reading the works of Lord Byron and he makes all sorts of references that I don’t get, however the average reader of his time would be familiar with the contemporary poets, military leaders, and even lawyers he mentions. I suppose if you’re writing with the aim of still being relevant a hundred years from now, you should avoid contemporary references as much as possible, but if you’re just writing for present day readers, feel free to mention people, slang, and fads that are currently popular.
I find it a fun exercise to try to write sci-fi from the perspective of someone in the future. Just like we don’t need cars explained to us, people in the future wouldn’t need to have faster than light travel explained to them. The average person of the future wouldn’t even know how it works, just that it does. When you write a story that takes place in the present, you don’t spend time explaining how an airplane works. It’s just taken for granted. Besides, any sci-fi explanation you come up with will quickly be dated.
So how much you tell versus show depends on the audience you’re writing for. If you assume your audience comes from the same culture as you, you can leave a lot unsaid. If you want people from different cultures (including people from the future) to understand what you’re talking about, you’ll need to explain a bit more. And the future happens faster than we think. When I was growing up, cell phones and even answering machines weren’t very common, so if a character missed a phone call from another character, you didn’t need to explain why, whereas if a character misses a phone call today, you would need to explain what happened (their phone’s battery is dead, they were somewhere without reception, etc.).