Emotions
reveal us, but we don't often reveal our emotions. Players on your
page are the same. They show us who they are via movement, quirks,
gestures, what they notice around them, their history, and many other
aspects--rarely through straight-out delivery.
So
a writer has to both observe and write the signals of emotion.
Characters who are well observed come alive for the reader.
But
we writers get lazy. Just as we take real-life friends and family for
granted--and stop seeing their uniqueness--we can fall into routine with
our characters. We copy characteristics in people we know, or we use
stock images for emotions without trying hard. Our observations grow
limited and (to the reader) boring and predictable.
This
creates what's know as the "flat" character. The antidote is to let
yourself really observe, so you can see around the stereotype and create
fresh, original characters.