Showstopper! The Improvised Musical offers a brand new song-and-dance spectacular at every performance. It opens with a brilliantly chaotic piece of comedy. A theatre producer on stage telephones Cameron Mackintosh and pitches him a new musical. Mackintosh answers and the producer invites ideas from the audience. ‘What’s the setting?’ Someone yelled ‘Late-night sauna’ at the performance I saw. The producer, without missing a beat, told Mackintosh that the show would be called, Sweat, Sweat, Sweat. If that was improvised it was world-class. The show develops along the lines suggested by the crowd and a number of hit musicals are parodied.

The audience, I suspect, enjoyed this more than me. The character of the producer faded away and his improvisational panache was forfeited. The sets and the costumes on stage appear rather drab and perfunctory because they have to be adaptable to any number of cues from the audience. The visual palette is dominated by red and black and the show looks as if it hasn’t quite left the rehearsal room. And though the performers sing superbly, they lack glamour and stardust. Showstopper! comes to the West End in October and its backers are hoping to recruit a fan base of addicts who will come back again and again. It may work. But I wouldn’t bet my last penny on it.

The Christians by Lucas Hnath looks at fundamentalism in America’s Bible belt. The show consists entirely of speeches given at the altar of a new church founded by the powerful and charismatic Pastor Paul. He boasts that he built up his congregation from nothing and turned the church into a wealthy and popular seat of worship. Having paid off his debts, he announces a new teaching. There is no hell, he declares, only heaven. The biblical term ‘gehenna’ refers to a rubbish tip on Jerusalem’s outskirts where waste would burn night and day, hence ‘the fires that never go out’. But the assumption that sinners and unbaptised souls will be condemned to torment in the hereafter is entirely unwarranted. This heresy splits the church. Paul’s colleague, Pastor Joshua, departs and forms a rival group. Some congregants, like Sister Jenny, ask Paul to confirm that no soul dwells in everlasting fire. Correct, says Paul, even Hitler is in paradise. He then faces a reckoning with his wife, who wants to curtail his dogmatic arrogance.

Lucas Hnath draws his characters skilfully and with sympathy: the admirable but power-hungry Paul; the decent but indignant Joshua; the needy yet intellectually fearless Sister Jenny. It’s rare to find a play that analyses Christian feeling with such high seriousness and sympathetic intelligence. Bible scholars will be delighted by the close textual analysis of the New Testament. Students of Attic drama will recognise that this grand and slow-moving play uses the same model as the works of Aeschylus and Sophocles: stationary actors making long, weighty speeches about events that occur offstage. Well worth a look. It transfers to the Gate, in London, next week.

John Lloyd: Emperor of the Prawns is an hour of metaphysical rumination performed by the producer of Blackadder. He includes stories about his war-hero father and his great friend Douglas Adams, but the heart of the show is philosophy. Lloyd pursues it in two senses: the love of wisdom and the quest to discover the purposes and origins of the universe. Anecdotes and quips, some elderly, keep the audience attentive. An Irish builder is asked by a pompous Hampstead architect to distinguish between ‘girder and joist’. ‘Well,’ says the Irishman, ‘Goethe wrote Faust and Joyce wrote Ulysses.’ Lloyd discusses the mystery of existence and claims that scientists can’t agree on a working definition of life that would usefully distinguish between a dead and a living rodent. He’s sceptical of the big bang theory and believes the universe began not with matter but with mind or consciousness. He suggests a symmetry between awareness and the concept of number and argues that ‘one’ and ‘zero’, like ‘on’ and ‘off’ or ‘being’ and ‘non-being’, are the biological equivalent of the binary system. As it happens the binary system is the conduit for all the digital information in the universe. He also engages in ethical questions and gives us his personal version of the commandments. ‘Be kind. Do what you know to be right. And bloody cheer up.’ He’s not immune to the temptations of evangelism and he jokes that he might become an archbishop, ‘and get someone to run up some nice robes for me. And a hat.’

It’s hard to imagine a better way of spending an hour of your life than with this gentle, funny, wise and ceaselessly inquisitive thinker, who happens to have inspired some of the best TV comedy ever created.