Sartre is talking about the infant Flaubert in particular but also human relations generally. Gustave mimes - or performs - emotion in order to provoke it in the parent:
A child who is bored suddenly takes it into his head to throw himself into his parents arms; it is not the overflowing feeling of tenderness that provokes him to it, but the future joy he will experience with their kisses.But 'overflowing tenderness' performed, perhaps copied from a sibling or parent, can later become 'overflowing tenderness' experienced. The performance was the shape, the outline, into which the actual emotion can grow.
I remember as an infant I would very often mimic the facial expressions of my parents. Not for fun or spite. If I saw anger or concern I did not full understand these emotions, but hoped, by getting into their skin, by re-producing their external form, to feel a version of them. And I think it worked - and still does. The facial or gestural outline of an emotion will carry that emotion in its wake.