The minute you introduce your protagonist, if they are not an actual person, living or dead, you are writing alternate history.
That means that, to some extent, all fiction has the issue of presenting the world it takes place in to the reader.
I hear some of you mainstreamers now: “But it happens in a town like I live in and everyone knows –”
Except the people who don’t live in towns, who live on farms or ranches, or in towns in a different part of the country, or who have never been outside the big cities. Except for very regional fiction, your audience ought to include people that don’t live where and how you do. If you live in a Maine fishing village, let the rest of us live there for a little while with you and your characters by presenting their world, not just the events in it. Let the readers in Phoenix and Gilroy, Lebanon and Atlanta, Hilo and Adak and London and Rome, see what your world is like.
This is the business everyone must attend to in fiction, of world presentation. Some people say to not bother telling or showing the reader anything they don’t absolutely need to know for the plot — but I wouldn’t read a book like that. Would you want to read a novel where the writer never made you feel the air of the day, the sounds of the place, the scent of the garden or the garbage, the light through the window? You don’t have to do that constantly, but you need to do it enough that your reader isn’t left trying to fill in everything for themselves — which may do very wrongly — or Hanging In Formless Space. HIFS is a good way to get your reader to leave. They have to work too hard to make the story real and they are getting none of the rewards of being led into some place they either have never been or can only go through books.
As well, twenty years from now, O mainstreamers, will any reader be able to fill in the town you live in that will be possibly very changed, even if they live where your house used to be? You don’t have to describe how cell phones work, but give us a little visual, or even the perfect detail, like the way they heat up your cheek and after a long call you can rub them on a sore muscle for a little relief. Think of how, in the 2013 NaNoWriMo Reference Desk, people are asking for background on the 1990s, as already a foreign place (anyone having a cellphone was a rarity, f’rinstance, but pagers were becoming popular to let you know who to call when you could get to a phone).
So don’t leave your readers HIFS, whether you’re doing mainstream, mystery, historical fiction, and especially not in fantasy and science fiction. Give them grounding, but don’t drown them in details. It’s a pretty thin line between, and some readers can take or need more detail than others. You can’t tell which, so while you should go for The Telling Detail, don’t give us too little.