I've encountered men and boys who are seemingly unable to sing in falsetto. They sing as high as their chest voice will allow, straining at the top, but are unable to slip into falsetto.
Presuming this isn't a physiological impossibility for them, how would a voice teacher approach teaching falsetto to them?
I've framed the question around male voices, because I'm allowing that men and women are trained differently, and my particular encounters with this limitation has only involved male voices. However, there's no reason answers can't address both.