Salesmen, TV presenters, and primary school teachers, amongst others, can have a mask in the form of a rictus grin: a smile or laugh that pervades every locution like disruptive aniseed overriding the actual flavour of the food, a patina of enforced cheer to encourage the customer to buy, the viewer to stay glued, the child to enthuse itself in the subject matter.
Yet the product is bland, the show is vacuous, and the lesson is a dead branch floating in the water.
I've started to watch out for the temptation to allow little laughs to leak into my voice when nothing I've said is funny.