Opening the book…
People do not arrive as workers; they arrive as whole lives with a job attached. A sick parent, a new baby, a bad diagnosis, a mortgage that just got heavier — these explain most of what looks like sudden underperformance. If you only know someone's output, you will misread them exactly when they most need reading correctly.
Ask questions that have nothing to do with deliverables, and remember the answers well enough to follow up weeks later. Notice when someone's pattern changes and treat it as information rather than a fault. When you adjust a deadline or reassign a task because of what you know, do it quietly and without making them explain themselves again.
This is not licence to excavate. If someone keeps their life private, that is their answer, and pressing for detail turns care into intrusion.