Opening the book…
Firings, rejections, demotions, and the aftermath of a serious mistake are the moments people remember with total clarity for the rest of their lives. Your power is at its peak precisely when their defences are lowest, which is why the temptation to be efficient, or to make a point, is strongest. How you behave here is what everyone else in the room learns to expect from you when their turn comes.
Deliver hard news yourself, in person, early in the day, with the reasoning given straight and no padding. Let them keep their exit intact — no audience, no debrief to others about what was said, no version of the story that makes you look better. Ask what would actually help, and then do that thing.
Dignity does not mean vagueness. If someone has done real harm, you name it clearly; softening the truth to spare feelings protects you, not them, and leaves the person no accurate picture to work from.