Pantomimes are plays based on a well-known story — often a fairy tale — which are given a bawdy twist. The audience is expected to join in throughout, shouting as loudly as they can.

Foreboding music begins. A scary green witch announces her arrival with a cackle. It’s the opening of Wicked Witches, a British holiday-time play known as a “pantomime,” at a North London theater. But ...