In the book, SAVE THE CAT, by Blake Snyder, there's a method called The Pope in the Pool. Snyder refers to a script where back story was introduced while the Pope was swimming laps. So what did the reader focus on? The Pope in a freakin' bathing suit in the pool!
Very clever and now I can easily spot this method in movies/shows/books.
The most recent example is in the HBO show GAME OF THRONES (have you watched it or read the series, and if so, did you love it as much I did?). Sometimes back story was introduced amidst some *cough* racy or scary scenes. And it was brilliant!
In a nutshell, we need to sprinkle back story here and there amidst some action and dialogue, to keep the reader engaged.
But that in itself isn't easy. Not at all. But I have gotten better at it.
Here's an example from the *awesome* book, A SCARY SCENE IN A SCARY MOVIE by Matt Blackstone. The MC, Rene is talking to his teacher, Mr. Head. I'll highlight the back story in red:
That's when Mr. Head hung his head and let out a low, guttural moan--the sound Phil used to make after coming home from work. He said it's "the sound of a man staring down a dead-end career all the way to the grave." Mr. Head, the second dying man I'd met, rose from his chair.
So the story moves along with Mr. Head and at the same time we learn something about Phil (his father). It's just a sentence or two sprinkled into the action and that's all we need, for now.