Think about how your fictional world is run. If someone asked you a question about it, could you answer them? Do you know enough about the climate, the history, the reason your world is run the way it is?
Consider life from the point of view of the ruling classes and of the “peasants”. Can make good comedy/tragedy but you can take the same event and get two differing but equally valid views. It can foreshadow troubles to come because of the differing perspectives. Equally that can explain why there are historical differences between say in my world’s case the ruling royals and the Witch’s family.
I find physical descriptions of characters hard too. I tend to focus on a trait - in Eileen’s case, stubbornness - and find that a useful starting point. I also tend to hear voices, with images of what characters look like coming later. Does it matter? I don’t think so.
As long as you’ve got a distinctive view of what your character is and how they’re likely to behave in any given situation, the rest will follow. Some authors prepare full bios for their characters. I don’t but there are no rights and wrongs here. It’s a question of finding the right method in character generation, the one that works for you.
Ensure dialogue is distinctive for your characters too. L’Evallier never contracts his speech whereas the Queen generally won’t but when under pressure the odd abbreviation will slip out. As for Eileen, her speech is as direct as she is. I can see the point of swearing to show a character under stress, to show their background, to show some of their attitudes but don’t like too much of the stuff. I treat swearing like paprika or chilli - only use a little.