Try writing about a secondary or supporting cast member as if the story was their story, about their life, and the events unfolding are interacting with them as part of their own character arc. You may not know how their story ends or begins, but knowing what they want, fear, feel, and how much of the “main plot” they care about or know about is instrumental in making their moments within the story three-dimensional.
Ideally everything in any story should be a walk through a solid world where all the scenery has functional backs, the metaphorical toilets flush, and the rest of the population of the earth continue with their own dramas that have nothing to do with your protagonist and antagonist, and keeping in mind the fluctuating patterns of reality helps give weight to your own story. The “background” feeds into the foreground and nudges it in unexpected ways. It may offer up subplots. It may change the direction of the story entirely. But what it does more than anything else is raise the story from the flat of the book and make it into something which lives and participates in its own world.
The writing exercise version of this is: write about a secondary character from their point of view.