When a London theatre decided to warn potential audiences about strong language, sexual references, grief and death in its latest production, the play’s celebrated star didn’t hold back. “I think it’s ludicrous,” Ian McKellen told Sky News. “I quite like to be surprised by loud noises and outrageous behaviour on stage.” Frank and Percy, a play about two retired men who meet on Hampstead Heath, opened last week with McKellan, 84, co-starring alongside Roger Allam. The warning issued by the theatre, the Other Palace, was in line with a growing trend to alert the public about potentially upsetting themes in...