You will try to make your reader care about the character you’ve created. You’ll do this by telling your reader all about the character’s background, family, where he lives, his personality, clothing, job, and who knows what else. You’ll lay all of this out for your reader in great detail. You’ll do this at the top of the first page. You’ll do this before you tell them the story. You’ll do this because they have to know. They have to know all of this before they can appreciate what you have to tell them.

Go ahead. Do this. Tell them all of it.

Now, delete it.

There. You’ve gotten that out of your system.

Now start again. Show the reader your character in the heat of the moment, dealing with conflict, caught in the middle of some action. Do this starting in the first sentence.
Do this well and your reader will care about the character you’ve created while knowing nothing else.

How do we know that this is true? We know because before we were writers we were readers. And this is why we care about characters. This is why we’ve cared about characters for years.

If you still want proof, go to the library and check out Cormac McCarthy’s No Country for Old Men.
You’ll meet Llewelyn Moss at the top of page one hunting antelope. It will not be until page 20 that you’ll find out he lives in a trailer with a woman. And even then you won’t know anything else, except they aren’t part of the social elite and Moss isn’t a sissy. On page 12 Moss will find a bunch of dead guys and on page 17 he will find another dead guy and a bag full of cash. And this stuff matters. It matters because back on page one, during the hunt, you started caring about this guy. You didn’t start caring on page 20.

As Ben H. Winters said over lunch on March 10, 2016 at LePeep’s on 71st St. in Indianapolis – always put backstory in later, and if possible, never.