Treat your first read on someone like a pencil sketch. Keep the eraser close.

Early impressions are drafts, and their usefulness depends entirely on being held that way. You gather a lot in the first few minutes, and almost all of it is inference from a very thin sample. The problem is not that you form a picture. It is that the picture hardens before the evidence arrives.

Notice when you have already decided about someone and how little you are actually working from. Look for the thing that would contradict your read rather than the thing that confirms it. A sketch you are willing to redraw is far more accurate than a portrait you defend.