Cabin crew and call-centre staff are required to appear cheerful regardless of how they feel. Why is this emotional labour associated with burnout more than physically demanding work?

D. Because keeping the display apart from the feeling is effortful

Surface acting is suppression with a uniform on: the feeling runs, the display is held, and the effort is continuous. Deep acting, actually reframing the customer, costs less, which is why the distinction is more than semantic and why training that only enforces the smile makes the problem worse.