In my experience, characters either come to you suddenly and miraculously, or they arise as you write the story. If a character isn't coming to mind after a short period of pondering, bullet point what purpose they need to serve to either the plot or the other characters, make the most barebones cookie-cutter one-dimensional character stand-in possible to serve as a "placeholder" in scenes and plot, and get to writing. As your story takes form, those less filled-in characters will too. Edits and rewrites will likely bring your characters more forward as well.
This advice behooves anyone who is writing anything less than the most plotless, character-centric story, like Normal People.