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It's both, actually.
Emotional expressions, like smiling, usually happen as we are feeling an emotion.
They give away what we are feeling.
Some facial expressions are difficult to create, unless we're actually feeling the emotion.
Genuinely sad facial expressions, for example, are very difficult to create.
It involves the inner corners of the eyebrows pointing upward,
so that the eyebrows form a upward-pointing triangular shape.
There's also a wrinkle in the area between the two eyebrows.
There are few people who can actually create a genuinely sad expression, on command.
In order for the sad expression to be real, some sadness is usually involved.
However, we can deliberately bring forth emotions in ourselves,
by forming our facial muscles so that they create a genuine emotional expression.
There are many kinds of smiles. Some are real, and some are fake.
If we are able to move the muscles on our faces in a way that forms a genuine smile,
we'll actually feel a bit of happiness.
We won't feel INTENSE happiness by doing this. But we can feel SOME happiness.
Edgar Allen Poe was someone who knew this well.
"When I wish to find out how wise, or how stupid, or how good, or how wicked is anyone, or what his thoughts are at the moment, I fashion the expression of my face, as accurately as possible, in accordance with the expression of his, and then wait to see what thoughts or sentiments arise in my mind or heart, as if to match or correspond with the expression."
In other words,
Edgar Allen Poe knew that emotions could be deliberately created, by forming the correct facial expression.