There’s a dark turn in the third installment of the Classic Thriller Season in Nottingham this week. Ira Levin’s Veronica’s Room, which is at the Theatre Royal until Saturday, is a true psychological thriller with enough twists and turns to keep the audience in suspense from the start until the very end. It all starts innocently enough – as they all do – when a newly-dating couple, played by Susan and Larry, agree to do a favour for the McKays, played by Susan Earnshaw and Jeremy Lloyd Thomas. The McKays are housekeepers of the Brabisant mansion and Susan – a young Boston college student – loo…