It sounds very limiting. I see posts all the time claiming that it’s stupid advice, because then no-one would ever write fantasy, historical fiction, space opera, etc. We’d have nothing but contemporaries about medical billing and timeshare sales or some other very pedestrian occupations.
Of course,”what you know” isn’t really limited to “what you’ve personally experienced”. If you have written a long series about a fictional fantasy kingdom, you presumably have a canon. You know what its geography and topography are like, how people dress and eat and worship, what weapons they use, if there is magic in that world and what rules that magic operates by. It may be imaginary, but you know it.
Same thing with any fabricated universe, or with fictional realities that intersect fantasy with a more tangible world that can be researched and have a more empirical knowledge base.
Sometimes you can’t get it perfectly right, as in historical fiction. You can try, but but it’s never going to be perfect. And yes, writers are working in Romance Land, or Mystery World, or some other variant of real life where things do not operate by exactly the same rules they do in this reality. In the end, it’s a work of fiction.
That does not excuse sloppy.
It is harder to create a plausible suspension of disbelief if you play fast and loose with the laws of physics. Or perpetrate the kind of anachronisms that anyone who was awake in fifth grade social studies might catch. Or have an MC go straight from their local community college to some profession where advanced degrees are required. Or drive from New Jersey to Chicago in ten hours. Or so on.
Yes, it’s an imaginary story. It isn’t required to be completely realistic. What it does need to do is follow its own rules. Or rather the ones of the universe it’s set in. The reader doesn’t need every nitty-gritty detail (some of the best world-builders out there do it in a sentence or two–think Jesse Hajicek or Ginn Hale), but the author needs to know where that universe diverges from the one we occupy, in detail, and convey it to the reader where it matters.
If it’s essentially the same everyday world we’re in, follow those rules. Do a little research. If your character is calling from a bus station at 10pm, find out if it’s open. Sometimes you have to use license, such as if there’s any likelihood he’d find a payphone, but if you have a reader who regularly uses that bus depot and knows they close at eight, that little detail will likely pull them out of the story. Ruin their immersion in the world you have created.
The more implausible details you have, and the broader the audience that might notice those details, the more likely that is to happen. There are readers that don’t care, but you’re probably not writing exclusively for those readers, and honestly, you should care. Your craft, your world, your characters.
One of my major pet peeves (and I’m bugged by pretty much any discrepancy in fiction I catch) is how many writers get New York City wrong. New York is probably particularly prone because it’s iconic, hellaciously expensive, and more than a little idiosyncratic, although I suspect authors do it to San Francisco nearly as often, and in similar ways, although I’m not going to be as aware of that since I’ve never been there (hats off to Amy Lane and her Going Up for her representation of six adults sharing a smallish two-bedroom apartment).
Yes, it’s fiction. I write in Romance World, and my character can get a high five/low six-figure job out of law school (since he graduated close to the top of his class at an Ivy League school) and a studio in the West Forties. What he can’t do is make partner in six months, or afford a penthouse, or park his car in front of his building and drive it cross-town to work every day.
The difference is the possible (if not necessarily the probable) and the impossible (assuming that Manhattan island is not twice the size it is in our universe, with half the population and a major shortage of attorneys–and if these things are so, or some other circumstance exists that explains one or all of these wild unlikelihoods, I kind of need to clue the reader in as to why). Yes, I know there’s a certain kind of escapist literature that uses unbelievable material success as its stock in trade, but even then, I think it’s improved by a nod to reality; some rationale for that wild success, and if you’re not writing that particular brand of escapism…
I’ve never thought writers have to (or should) stick to personal experience, which is a terribly limited definition of “what you know”, but when you’re not sure (or occasionally even when you are–I’ve found things I was positive of that turned out to be wrong) research is your friend. Google maps, Wikipedia, vetting it with someone with more direct experience, etc., etc.
So yeah, write what you know, but you probably know more than you think you do.
Addendum: A reader pointed out that someone with a disregard for speed limits probably could make Chicago in ten hours from some points in North Jersey. Which kind of proves the point about even when you’re sure…