I’ve found that writing little scenes that don’t need to be in the story can help with this. Like writing the “they woke up, did their routine, went to work” scenes can help you get to know them, see them as any other person, which can help when trying to write their story. Obviously this would take forever to do with every side character, but with my main characters it helps a lot. And you can even write random interactions between side and main characters, which has also helped me.
Your characters aren’t just plot devices. They existed before the story started, and they’ll exist after it ends. Give them history, quirks, and contradictions. Maybe they always order the same coffee because it reminds them of home. Maybe they pick fights because it’s easier than being vulnerable. Maybe they love thunderstorms because they grew up listening to the rain through a broken window. The best characters feel alive because they have little pieces of reality stitched into them.