Great writers love shortcuts.
No, really. Given that folks like George RR Martin can take eons to finish a book, that may seem counter-intuitive. But the truth of the matter is that writers love things that enable them to get down to the nitty-gritty and write our stories.
Which is one of the reason that writing so-called "genre fiction" can be so hard - you end up having to spend simply ages creating an entire reality for your characters to exist in. And most of us (with apologies to authors like JRR Tolkien and Brandon Sanderson) are not really in love with our settings - we are in love with our characters, and want to tell their stories.
And the simplest way to get down to telling your story is to "write what you know" - set your book in a "real world" setting.
The Really, Real World
It applies across so many stories. If you want to write a romance novel, everyone knows what a "rose" is. In your Western, stick with "horses", and you will not have to spend time and mental energy describing the ecology and development of the "6-legged Runner Beast" or whatever. Your YA novel can feature a "coffee shop" as a central meeting place, and your reader is free to place their own interpretation of a local coffee shop in their mind and move forward without a whole lot of mental gymnastics.
The truth is that utilizing a "real world" or nearly real-world setting can save a tremendous amount of time and energy that can then be spent working on the really meaty stuff of your story - characters and conflict. Using the sights and sounds that you are already familiar with can not only save time, it can also add depth to your setting and description. As always, your story needs to stand by itself - no amount of realism and description is going to make up for one-dimensional characters doing unimportant things.
But taking your strong characters and making them real still requires some work beyond setting them in an environment that already exists.
Show, Don't Tell
While using the shortcut of writing for the real world, it becomes even more important to have nuanced descriptions of the environment your characters are moving through. Saying your character "waited in the Emergency Room" is a great example. While most of us know exactly what is meant by that description, you've passed up a great opportunity to understand your character better. How about, instead: "She waited in the lobby, under the wan, flickering fluorescent light, breathing in the almost overpowering smells of astringent cleaning fluid and unwashed bodies. Her hands clasped then released the handle of her purse over and over again as she waited for someone wearing scrubs or a white doctor's coat to come and let her know her brother's condition." Most of us have been in a similar situation, and we can identify with the character and understand her emotional state, by placing ourselves in her shoes.
The danger in writing what is familiar to us is that we can use too much shorthand, and under-explain. Sights are important, but so are smells, sounds, textures, even tastes. These are all golden opportunities for immersing your reader into the environment where your characters interact with one another.
One of the best tools for adding strong descriptive elements is the "field trip." Once you have sketched out the general outline of the settings of your Real World novel, take a day and visit the types of locations you will be using in your book. Spend some time in the business district at rush hour. Hang around the food court at the mall. Take a look at upscale neighborhoods or lower income areas. Take pictures and make notes, if appropriate. If you are writing from a historical perspective, take a look at images on the Internet. Watch a movie or read a book which shares your setting. But take note of those descriptive elements that speak to you most strongly - those are the things that you will be able to communicate to your readers most fluidly.
But make sure you are performing your due diligence, of course.
Make Sure It Makes Sense
Another potential point for excellence or failure is to make sure you have at least a passing familiarity with what you are writing about. Nothing breaks immersion faster when reading than an author demonstrating that they obviously don't know what they are talking about. Don't have your characters boarding a 747 in 1923. These types of "misses" drive readers crazy.
The real problem is that the readers who are being driven crazy are usually fairly knowledgeable and well-read ones - the exact reader you want your book to appeal to because they are more likely to communicate to other readers what their opinions are about the books they are reading. While it isn't necessary to be an expert in medieval architecture for your historical romance novel, you probably shouldn't represent ranks full of male and female soldiers during the Norman Conquest. Be aware of who you are writing for.
Getting this kind of information is usually no more difficult than sending an email and making an appointment. I've gone on ride-alongs with police officers, chatted with firearms instructors, and talked with historians who specialize in urban planning and development. Most people are delighted to share their experience with you - usually for the cost of nothing more than a beer or a cup of coffee. Acknowledging those who have helped you along in the credits for your finished product is, of course, recommended as well as a repayment for their courtesy. But the information is readily available, leaving no real excuses for not having at least a passing familiarity for the subject or setting you are writing about.
As you draw out to a macro level, ensure that your setting makes sense in and of itself. If you are setting your novel in New York City or Los Angeles, be as descriptive as possible - show the reader where these characters are, and how they are interacting with the world around them. Even if you have no personal experience with the cities or countries in question, some time spent looking at images online and perusing books at your local library can do wonders for broadening your knowledge and giving you story hooks to hang plot elements off of.
What do your New Yorkers think about crime and politics? Do your Southern California residents have opinions on how long it takes to get from one point to another? Also, do your best to include a bit of nuance in speech patterns - a rancher from Texas will communicate differently from a socialite from Boston. Netflix and Hulu can be your friends here in giving you hints and clues.
Lastly, if you are going to be inventing your "real world" setting out of the whole cloth, make sure it makes sense. Before you plop a castle down in the South of France or a city in Eastern Nebraska, ask yourself some questions. How did it come to be there? What is its history? How is it supporting itself currently? A booming farm depot city is going to feel very different from an old, forgotten railroad community largely forgotten by time. Make sure that your world is internally consistent.
Familiar Is Not Insignificant
It always pays to remember that your characters are moving through a world filled with stimuli. The more they interact with the world around them, the more real they will seem, and the more invested you readers will become. Make sure you are taking some of the time you are saving by not creating fantasy maps and Space opera backstory, and invest it in making their world as similar to the one you exist in as possible.
Your characters should never exist against a flat black backdrop, but instead should be in a world filled with movement, excitement, and conflict - even when the conflict does not directly involve your cast. Take the opportunity you are given by your intimate knowledge of the place where they live and make it sing with reality. Make your readers gasp when they come up for air, having been so immersed in the world that you've created for them that they have momentarily forgotten the one around them. That is the ticket to successful writing.