And that is -- the backstory you already have plotted out. Suppose you gave your hero's mentor a tragically murdered wife. And then you do a prequel about the mentor. You have to put in the wife. And you have to murder her. And while you can evade the matter, you really ought to develop her and make her sympathetic and have it be as tragic as you hinted at. Because in the backstory, you only hint at things that will be much more dramatic, and excruciating, in the story itself, but we want the full scale of it that would justify the effect of it on the mentor, later.
Or the villain treacherously attacked his own men when he realized they would not support his revolt. That will establish him as a villain of the first order of magnitude. In the prequel, however, you have to show it in action. And develop the betrayed men as characters so we feel the extent of the villainy. And not give them any means of escape that preclude the events in later books -- and, in order to establish the depths of his villainy, probably none at all. If you do that, you will have done the aesthetically correct thing, and I will approve, even if I want to murder you. But if you wimp out. . . .
For some reason, the one point at which writers wimp out is when they depicted a character in an unhappy marriage. In the prequel, you have to show how the character came to make such a stupid match. I have yet to read an aesthetically satisfying book in which I was convinced that, yes, the character -- the same character we see in the latter books -- really did marry such a ill-suited spouse.