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THE main challenge in adapting a popular movie for the stage is not so much to do with form as with audience expectation. Not long ago, I overheard someone coming out of Richard Baron's fine and funny staging of The 39 Steps at Perth Theatre, complaining - surprise, surprise - that it wasn't like the film.

And though it's possible the crowds gathering in Edinburgh this week for Kind Hearts and Coronets are there because they want to see Robert Powell and Colin Baker in the flesh, it's a fair bet it's because they enjoyed the Ealing Comedy from 1949 which starred Alec Guinness.

Adaptor and director Giles Croft nips this problem in the bud. The curtain rises, and we see a television screen. On it, the early scenes of the movie.

Lights up, cue Robert Powell to tell us that yes, there was the film, and yes, before that was the novel, but this - ha, ha - is going to be the true story. As Louis Mazzini, he remains on stage, our genial host, to narrate his own murderous ascendancy to dukedom.

Croft's approach is to lift both the lightness of the movie, and the darkness of the novel, producing a comedy that grows blacker as it goes on. The comic touches are fine, there's a playfulness to the staging around the first murders, even if it's surprising how little fun is made of Colin Baker playing all of the victims. It does appear, however, that the adaptation secretly wants to be a serious study of evil, akin to Macbeth, Don Juan, or Dr Faustus, and by the end, material well suited to black comedy proves too flimsy to support such weight.