Wittgenstein said that the path to truth can be traced through an investigation of error. Taken liberally, he encourages us to take note of differences and similarities in a concept by considering a diverse range of cases.
If the meaning of adult facial expression is to be understood, it may therefore be benefical to consider the experiences of adults with facial disfigurement. And if you want to understand pain facial expression, it may be worthwhile to consider the case of adults and children with congenital facial paralysis.
Comparing radically different cases may serve to highlight forgotten niches of concepts.