https://en.wikipedia.org/wiki/Ian_Cognito
Also apparently this is a thing that happens: https://en.wikipedia.org/wiki/List_of_entertainers_who_died_...
Cracking jokes one second, and then quietly down the next.
https://www.sunderlandecho.com/news/night-sid-james-died-sta...
Besides getting killed by something or someone else, it seems heart attacks are a very common cause of death while performing. I wonder if it's due to the stressful nature of the situation in general.
This was in Germany and they were wearing local uniforms. That made it seem rather authentic.
I remember being somewhat certain that it was an act, and there were no indication from the audience that anybody fell for it (panic, leaving, yelling "I'm a doctor").