How to give characters depth that have little screen time

Dear scribes,

I repeatedly got the note that my supporting characters are too two dimensional. Is there a shortcut of giving them development without stealing time/lines from the protagonists? In my story I have already two main protagonists that need so much backstory and development. And even there it never seems enough for the readers. I mean there is a reason why you have one main protagonist for the main arc who gets the most dev, right?

Are there tricks how to do this? Tools? I mean aside from giving them background story, wounds etc. Any examples from movies where someone with little screen time could be viewed as three dimensonal? Or just examples of films where characters are considered very deep even protagonists vs films where they glaringly lack depth?

Sometimes "deep" is such an abstract word. I lose track of what it actually entails when worrying about all the other parts of the story. Is it about motivation (the "why"), moral choices, wounds, backgrounds, little ticks? Is it about change? Should I give all characters an arc, no matter if main protagonists or not?

What do you as readers look for concerning character depth?

Thank you :)