How do soft and hard edges make a painted face feel alive?
Short answer: Edges control where the eye goes and how form turns: hard edges draw attention and describe sharp changes, soft or lost edges let forms turn away and melt into air. A portrait with a few carefully placed hard edges among many soft ones feels present rather than cut out.
Three kinds of edge
Hard edges are sharp transitions. Soft edges blend gradually. Lost edges disappear, where a form and its background share the same value.
Why it matters for likeness
A face painted with hard edges everywhere looks like a cut out paper doll. Softening the edges where the cheek turns away, where hair meets background, or where a shadow falls, lets the head sit in space. Saving the hardest edges for the eyes or a highlight tells the viewer where to look.
A simple test
Squint at your painting. Wherever an edge stays sharp while squinting, ask whether you want the eye to land there. If not, soften it.