A quick tip for adding more depth to your world and characters: show how your background characters relate to each other, not just your protagonist.
I've been noticing lately in a lot of novels that background characters tend to never interact with each other, only with the POV character. Even in group settings where they're standing around talking, they only address the protagonist. Sometimes they may echo each other, but rarely do they engage each other unless there's some animosity and conflict. There's little indication that they know each other, or talk to each other at other times, or have lives outside of the time they spend with the protag. There's little sense of shared history, camaraderie, interpersonal relationships. In groups that have recently formed within the story, characters tend to remain islands and only react to the words and actions of the protagonist. (Or perhaps they form small subgroups, but each of these groups functions as its own island, often with one leader and a few mostly-mute followers.) I find this unrealistic.
In any group, individuals are going to interact with every other individual. They are going to develop relationships and share experiences that shape their opinions of each other, and these things might happen (or have already happened) when the POV character isn't around to witness them. But they should still have an effect on how these characters behave toward each other when the protagonist is present, and adding little details to show this adds to the characters' personalities and the overall depth of the story.
Granted, you may not want to spend hundreds of pages fleshing this out. If you're using a limited POV, your protagonist obviously won't be proxy to the fight X and Y had or that shared coming-of-age experience between A and B. But they would still notice the little things, like how Y scoffs at X's suggestions or that joke about stew A tells that always makes B snicker. These are the things that help make A, B, X, and Y into fleshed-out people instead of interchangeable cardboard characters, even if they are minor characters. Of course, you don't want to bog down the story with details like these that aren't relevant to the overall plot, but in appropriate amounts, they can add a lot of atmosphere to the story. Experiences and relationships shape who people are, and providing hints of the ones your background characters have had that don't relate to your main character can help make them feel real.