By the time Helen Thomson bursts onto stage in Sydney Theatre Company’s upcoming production of The Unfriend, audiences will already be on eggshells. Her character, Elsa Jean Krakowski, is an exuberant American widow who befriends a respectable, middle-class English couple on a cruise. She’s gregarious, generous, impossible to shake and an unrepentant right-winger. She may also be a serial murderer.

It’s exactly the sort of role Thomson relishes. “Think Mary Poppins,” she says mischievously, “but scary.”

Helen Thomson in the hero image for Sydney Theatre Company’s production of The Unfriend. Photo © Holly Ward

The Unfriend is the first stage play by British writer Steven Moffatt, the celebrated showrunner of Doctor Who and creator of Press Gang, Coupling, Sherlock and Douglas is Cancelled. It became a West End hit by skewering one of Britain’s most enduring national traits – the inability to offend anyone, even when common sense demands it.

It begins with Peter and Debbie politely inviting Elsa to visit after meeting her on holiday. By the time they’ve googled their houseguest and uncovered some deeply alarming allegations, it’s too late to rescind the...